Previous studies had estimated that repeal of mandatory helmet wearing laws resulted in changes in the number of fatalities ranging from nearly 40% to essentially zero. This study was performed to determine more definitively the influence on motorcyclist fatalities of these repeals. After repeal, motorcyclist fatalities increased more in the states which repealed their laws than in those which did not in 24 cases out of 26, with the average effect being 28±4%. It is therefore concluded that repeals of mandatory helmet wearing laws for motorcyclists were followed by substantial increases in motorcyclist fatalities.