Serum from injured automobile and motorcycle drivers treated at a trauma center was tested for delta-9-tetrahydrocannabinol activity to determine precrash marijuana use. From June 1990 to March 1991, samples from approximately 20 automobile drivers per month and all motorcycle drivers were available for testing. Also, toxicology screens were performed for ethyl alcohol, cocaine, and phencyclidine (PCP) among the driver groups. Six (2.7%) of the 225 automobile (AUT) drivers and 34 (32.0%) of the 106 motorcycle (MTC) drivers were THC+ (p < .001). Compared with a prior study, the THC+ rate decreased significantly from 31.8% among AUT drivers (p < .001) but had not changed significantly from the 38.6% rate among MTC drivers. Positive toxicology rates were higher among the 261 MTC drivers compared to the 1,077 AUT drivers tested for ETOH, CO, and PCP, being 47.1% vs 35.2% (p < .001), 5.0% vs 8.0% (p < .08), and 1.5% vs 3.1% (NS), respectively.