Marijuana use prior to injury was determined prospectively in 1023 patients injured as the result of vehicular (67.6%) and nonvehicular (32.4%) trauma. Most were men (72.8%); most were 30 years of age or younger (58.4%). All were admitted directly from the scene of injury. Serum delta-9-tetrahydrocannabinol activity was ascertained using a radioimmunoassay. Activity of 2 ng/mL or more was detected in 34.7% of subjects. Blood alcohol determinations were made in 1006 patients; 33.5% were positive. Marijuana use among vehicular and nonvehicular trauma victims was not significantly different. Marijuana use was higher among those 30 years of age or younger and among men. Vehicular crash victims consumed alcohol more frequently. Use of marijuana and alcohol in combination (16.5%) was highly significant compared with marijuana alone (18.3%), alcohol alone (16.1%), or neither drug (49.1%).