New York became the first state in the U.S.A. to enact a seat belt use law for drivers and front seat passengers. Twenty-six other states and the District of Columbia have passed similar legislation since July 1984. Seat belt usage has increased from two- to fourfold under a mandatory regime. Preliminary data document benefits in reduced serious and fatal casualties and in insurance claims. Greater emphasis must be given to higher levels of enforcement if usage is to reach 75-80%. With substantially increased belt use, the effectiveness of belts, as well as their limitations, and of laws mandating their use, can be evaluated adequately. Also, other consequences of belts can be monitored more closely to determine their relative importance.