Abuse of cocaine is becoming a major problem among heroin addicts in Spain. Between 1987 and 1988, 75% of patients admitted as inpatients for detoxification from opiate dependence had consumed cocaine during the 6 months prior to admission and 25% had abused cocaine daily or several times/week. These cocaine abusers showed more toxicologic and psychopathologic problems than opiate addicts who did not abuse cocaine. The opiate addicts who also abused cocaine had begun using illicit drugs earlier and showed a higher frequency of anti-HIV antibodies. They also had more antisocial personality disorders and persistence of depressive symptoms during opiate detoxification than heroin addicts who did not abuse cocaine. Based on these findings, we insist on the need to develop different treatments for detoxifying patients with this dual addiction.