Data from patients visiting an urban psychiatric emergency service in California were examined to document incidence and patterns of substance use and ethnic differences among users. A total of 392 patients were randomly assigned to receive a drug screen (N = 198) or to receive usual care (N = 194). Forty-four percent of the mandatorily screened patients had positive screens for any substances: 37 percent were positive for any drugs, and 7 percent were positive for alcohol only. Cocaine was present in 62 percent of the drug-positive screens. Blacks were two and a half times more likely than whites to have positive screens for drugs and five times more likely to have positive screens for cocaine.