Between 1997 and 2005, the number of annual motorcyclist fatalities doubled. Motorcyclist fatalities now account for over 10 percent of all traffic-related fatalities. However, over the last three decades, states have generally been eliminating laws that require helmet use among all motorcyclists. This study examines the effectiveness of helmet use and state laws that mandate helmet use in reducing motorcyclist fatalities. Within-vehicle comparisons among two-rider motorcycles indicate that helmet use reduces fatality risk by 34 percent. State laws requiring helmet use appear to reduce motorcyclist fatalities by 27 percent. Fatality reductions of this magnitude suggest that the health benefits of helmet-use laws are not meaningfully compromised by compensating increases in risk-taking by motorcyclists.