During a 2.5-year follow-up of opioid addicts, we examined psychosocial antecedents and consequences of the onset and remission of cocaine abuse. Patients who never used cocaine were compared with those whose use increased or decreased along several dimensions of treatment outcome including drug abuse, legal, employment, family, social, psychological, and medical problems. Cocaine abuse had a marked impact on almost every outcome area except medical problems. Patients whose cocaine use increased during follow-up had more severe problems than either those whose use decreased or those who never used cocaine. Furthermore, the attainment of cocaine abstinence among abusers was associated with improved psychosocial functioning, whereas the onset of cocaine abuse was associated with increased problem severity. Compared with drug-free and detoxification alone treatments, methadone maintenance may minimize legal complications of cocaine abuse, but otherwise it did not significantly reduce psychosocial morbidity from increasing cocaine abuse. These findings suggest that treatment-seeking opioid addicts are vulnerable to wide-ranging deterioration when they become increasingly involved with cocaine but that with the attainment of abstinence many problem areas improve.