Cocaine is becoming a major drug of abuse among the general population and among opiate addicts. Reports from the early 1970s found that most abusers were older Black males with some antisocial characteristics. Cocaine abuse at that time was reported by about 17% of opiate addicts seeking treatment and by 7 to 11% of ex-addicts on methadone maintenance. However, that rate increased dramatically during the 1970s, and in our 1980 study of 533 addicts we found that 74% of opiate addicts applying for treatment used cocaine. It was the second most abused nonopioid drug after marijuana, surpassing alcohol intoxication. Although the mean number of days of abuse over the previous 30 days was substantially lower among the addicts on our methadone maintenance program (mean = 1.4 days, n = 120) than among the addicts applying for treatment (mean = 9 days, n = 204), the following associations with cocaine abuse were consistent in both subsamples. Cocaine abuse was more frequent among Blacks. It was associated with a variety of antisocial indices including Research Diagnostic Criteria antisocial personality disorder, number of arrests, and legal, family, employment, and drug abuse problems as assessed by the Addiction Severity Index and the Social Adjustment Scale. Several differences emerged between Black and White cocaine-abusing addicts, the most interesting being an increased rate of anxiety disorders among White cocaine abusers. Based on these associations, we offer several guidelines for treating cocaine abuse in opiate addicts.