New York State enacted the first safety belt use law in the United States in 1984. We evaluated the law by comparing all hospital admissions from motor vehicle crashes in Monroe County (Rochester) New York for 18 months prior to the effective date of the law with a similar sample after the law became effective. Police accident reports and hospital records were coupled and intensively reviewed. Motorcyclists, pedestrians and bicyclists (bicycle collisions with motor vehicles) became controls for the study. Seat belt usage was determined using Police and hospital record information. Analysis of the data revealed that safety belt use among patients hospitalized because of motor vehicle accidents increased from 11.2% before the law became effective to 53% after the law became effective. Hospital admissions decreased 11.9% among motor vehicle occupants and increased 2.6% among controls. The ISS decreased from 16.01 to 14.55 for motor vehicle occupants and increased from 14.77 to 15.11 among controls. Among subjects all injuries decreased except injuries of the spine and abdomen which increased in the post-law period.