This summer, when half a million bikers clogged the streets of tiny Sturgis, S.D., for one of the country’s largest motorcycle rallies, there might have been a small unexpected benefit for nearby patients desperately awaiting organ transplants.

Major motorcycle rallies are associated with increases in organ donors involved in motor vehicle crashes, according to a study published this week in JAMA Internal Medicine.

The authors theorized this could be because of the numbers of motorcyclists attracted to the rallies, dangerously congested roads designed for far less traffic, or riskier behaviors such as riding while fatigued.

Although they were careful to emphasize that policymakers should focus on reducing traffic deaths, the researchers wrote that they should also “anticipate and translate eligible deaths from these events into organ donations.”

The lead author, Dr. David Cron, who is a surgery resident at Massachusetts General Hospital, said he got the idea for the study when a colleague described working at another facility near a major rally in New Hampshire. “They used to gear up for these events and increase staff in the emergency department and plan for an influx in motor vehicle-related trauma,” he recalled.

Previous research has linked such rallies to increases in crashes and fatalities, but the new study found there were 21 percent more organ donors during major rallies than in the surrounding time periods, amounting to one additional donor for every two major motorcycle rallies.

Dr. Cron and the article’s co-authors identified seven of the country’s largest annual motorcycle gatherings, including the one held in Sturgis, accounting for more than 100 separate rallies in a 16-year period. Then, the researchers analyzed data about organ donations and transplants from the Scientific Registry of Transplant Recipients, which divides the nation into eleven administrative regions.

The researchers compared the number of people involved in motor-vehicle crashes who donated organs the week of a rally in one of those regions with the number of donors involved in crashes in four-week periods before and after the gathering.

But the researchers were unable to determine whether the organ donors were people who died in motorcycle crashes or in other vehicles. Overall, motorcycle crash deaths account for a growing share of overall traffic deaths. According to the National Highway Traffic Safety Administration, there were 5,579 motorcycle fatalities in 2020, more than any previous year and nearly double the number from two decades ago.

Laura Siminoff, a professor at Temple University and expert on organ donation who was not involved in the study, said it was “kind of obvious” that motorcycle rallies would be associated with greater numbers of fatal injuries. The more important implication, she said, was that hospitals and organ-procurement organizations were doing their jobs by facilitating donations from eligible patients.

In the United States, motor vehicle crashes are among the most common circumstances leading to organ donations by deceased people, which can only take place under certain conditions. Deceased donors have typically suffered catastrophic brain injuries but their other organs are sufficiently healthy to transplant. About one in 30 people who die in motor vehicle crashes ultimately becomes an organ donor. (Last year, about 6,500 living people donated a kidney or part of their liver, too.)

The researchers showed that the motorcycle rallies were not associated with upticks in the number of organ donors who died of strokes or drug overdoses, other circumstances that commonly allow for donations. During the weeks of the rallies, there were no observable changes in organ donation in non-neighboring regions, which made it less likely to be some unknown factor causing an increase in organ donation.

Kevin Myer, president of LifeGift Organ Donation Center based in Houston, said most victims of fatal motorcycle crashes suffered significant injuries that rendered their organs unusable, which he suggested might have reduced the number of donations that otherwise would have been possible at the time of the rallies.

Most states that previously required all motorcyclists to wear helmets have relaxed those laws. Of the states where the major motorcycle rallies in the study occurred, none have universal helmet laws, according to the Governor’s Highway Safety Association.

“While I respect folks who love motorcycles and stuff like that, they really should wear their helmets,” Mr. Myer said.

Although the apparent effect the rallies had on the number of transplants was small, there are scores of motorcycle rallies held across the county each year.

Still, Dr. Cron said, “it’s just scratching the surface of the massive organ shortage.”

Over 105,000 people are on the national waiting list for an organ transplant, and 17 die each day. Although over 90 percent of people have voiced support for organ donation, just half the population has registered to be an organ donor.