ILVERADO, Calif. ÷ On a warm afternoon in late summer, about 250 motorcycle riders have converged at the Saddleback motocross track here in Orange County, raising dust and an unholy racket as they test their skills on a torturous series of jumps, curves and whoop-de-dos.

Among them is a 17-year-old from Brea, Chad Robbins, an explosive jumper who was returning after an accident here a few nights ago. He had been trying a 110-foot double jump and landed atop another rider, injuring his back. But it was no big deal, he said.

"I've broken my leg, my ankle, my toes, both arms, my knees," he said. "Both wrists have rods in them. I've also been knocked out in a coma for two days." Asked why he was so willing to keep defying death, or at least paralysis, he said: "It's worth the rush. Motocross is my life."

Motocross ÷ racing and stunt riding with off-road motorcycles on dirt tracks ÷ has become one of the country's fastest-growing sports, fueled by the gladiatorial spectacle of arena races, the astounding jumps and flips of freestyle riders and what amateur and professional riders say is simply the intoxicating thrill of it.

But as the sport has grown, so has the number of injuries to riders ÷ sometimes to astonishing levels. Halfway through the 2002 season of Supercross, the professional arena-racing circuit, 18 riders, nearly half of the total field, had suffered serious injuries, including concussions and broken ribs, wrists, femurs and backs. The number of motocross-related emergency room visits has doubled in recent years, to around 53,000 in 2000, according to the Consumer Product Safety Commission. Nearly the same number occur with road bikes, which outnumber off-road bikes by three to one.

Annual sales of off-road motorcycles have doubled in recent years to nearly 500,000. Last year, the American Motorcycle Association, the sport's main organizing body, sanctioned 2,844 racing events, the vast majority of those off road, and this year that number had already exceeded 3,200. Supercross racing, held in arenas, is second only to Nascar racing in attendance, and on television, ESPN's Moto-X freestyle jumping events are a runaway hit.

The motocross phenomenon has made stars of riders like Travis Pastrana, an 18-year-old from Annapolis, Md., known as one of the most jaw-dropping motorcycle jumpers in the world. Mr. Pastrana has endured 12 operations, more than 30 broken bones and 10 concussions.

In 1998, at 14, he dislocated his spinal column from his pelvis after a failed 120-foot jump. In January, just after healing from a broken leg, he suffered a serious concussion during a Supercross event. Three weeks later, he had another. One concussion can cause permanent brain damage. In the 2001 season alone, Mr. Pastrana had five.

Yet despite his injuries, Mr. Pastrana remains unflinchingly devoted. "Every time I've gotten hurt," he said, "it has been worth it, and it has been my fault. After I separated my spinal column and woke up a week later in intensive care, the first thing I remember my mom saying was, `Are you sure this is worth it?' There was never a doubt in my mind."

Devotion like Mr. Pastrana's makes him a hero in the motocross world. It makes people like Rick Sieman furious. Mr. Sieman, 62, founded Dirt Bike magazine in 1970. He is an editor for off-road.com, a veteran of some 3,000 off-road races and a former Supercross rider who survived a broken back at an event in Los Angeles in 1979. He is one of the most outspoken advocates of safety in the sport, which he says glamorizes dangerous moves that young riders feel intense pressure to imitate.

"A 16-year-old kid goes out there and watches these pros, and his dad's with him, his girlfriend's with him, and he's saying, `I've got to do this,' " Mr. Sieman said in an interview. "Maybe if he's lucky, he just gets away with a broken collarbone."

"Professional Supercross racers have to sign their legal rights away," he continued. "They know that if you go out and get hurt, that's the price you pay for being Evel Knievel. But in those stands is a kid with a little motorcycle. He goes out with his buddies and the first thing they do is find some huge jump and try to impress each other, and the trend continues. The wagons keep hauling these morons to the hospital."

Doug Bueller, a spokesman for the American Motorcycle Association, said his organization was doing everything it could to address safety issues. He agreed that there were some problems with rider placement in races ÷ riders of varying skills mixing dangerously on tracks. But he maintained that the overall system worked. He asserted that the safety gear the association requires at its events ÷ high boots, helmets and gloves, though no torso protection ÷ was adequate.

Mr. Bueller said that although his organization sanctions thousands of events, it is up to individual race promoters to follow its guidelines. The association is rider-run, he said, and its riders' congress ultimately dictates the sorts of courses sanctioned. He said jumps and several other safety issues would be discussed at the upcoming congress.

He also noted that thousands of other races, including most amateur ones in California, are not run by the motorcycle association at all. This leaves a loose confederation of owners and promoters to determine track safety, a condition the association says it would like to see change.

To Rich Daly, a 51-year-old motorcycle racer from Union Springs, N.Y., the increasing dangers of motocross make safety reforms essential. He has called for a national licensing system, better inspections of motorcycles and helmets, and requiring chest- and back-protecting armor and a more thorough network of "flaggers" around tracks to warn racers of downed riders.

"Motocross has always been a fun sport," Mr. Daly said. "It's addictive, and it's always been very dangerous. But now we've gone to the gladiator level."

Principal among Mr. Daly's complaints are so-called double and triple jumps ÷ two or three evenly spaced ledges that the rider must either clear with one jump or roll over singly. A jumping rider who comes up short could come crashing down ÷ perhaps on top of a rider who decided to roll through instead.

To Mr. Daly, the solution is to "tabletop" the jumps, or make them flat at the top, and to paint a white line down the center to separate the jumpers from the rollers.

Before allowing his own children on a motorcycle, Dr. Jondy Cohen, a 43-year-old orthopedist and motocross buff from Modesto, Calif., decided to look into the actual levels of risk associated with racing.

In 1998, Dr. Cohen began compiling a record of injuries at races conducted by Jack Azevedo, a former professional rider and race promoter. He also consulted with Mr. Azevedo on what should be done to make his tracks safer. Since they began their study, Dr. Cohen says, they have reduced injuries by 50 percent.

Mr. Azevedo agreed to make the jumps more rounded, so that riders who did not clear them were not as apt to crash into the other side. He now keeps the track well stocked with emergency flaggers and has become quite strict about segregating racers by age and ability. Dr. Cohen, whose 12-year-old son, Logan, races at a track in Los Banos, said that he and Mr. Azevedo would begin asking area hospitals to have an orthopedist on call on race nights.