During the last 10 years, the number of motorcycle riders in the United States has risen sharply. The corresponding increase in fatalities observed during this time may be because of the increase in riders, or because the number of states that mandate universal helmet use has decreased. We examined the effect of the repeal of Florida's helmet law in July 2000 to test the hypothesis that the increase in fatalities observed after repeal resulted from an increase in the number of motorcycle riders.