Arrangements were made with coroners in four California counties to obtain blood samples from fatally injured male drivers of motor vehicles during part of 1982 and most or all of 1983. Of 789 young male drivers killed in motor vehicle crashes, 514 died on impact or soon after the crash and were thus eligible for inclusion in the study. Alcohol, the most frequently found drug, was detected in 70 percent of the drivers, marijuana in 37 percent, and cocaine in 11 percent. Each of 24 other drugs was detected in less than 5 percent of the drivers. Except for alcohol, drugs were infrequently found alone and were typically found in combination with high blood alcohol concentrations. The causal role of drugs in motor vehicle crashes was assessed by comparing drivers with and without drugs in terms of their responsibility for the crash. Alcohol was associated with increased crash responsibility, but the role of other drugs could not be adequately determined. 18 references and 6 tables