Levels of dysphoria and opioid dependence were assessed in 54 male patients with heroin addiction applying for drug treatment. During a period of naturalistic heroin use, symptom measures of dysphoria and of spontaneous opioid withdrawal reported by these patients were highly correlated. Upon admission to treatment, levels of dysphoria and opioid withdrawal were assessed before and after a pharmacological challenge with either 0.4 mg naloxone or placebo. Signs and symptoms of opioid withdrawal and symptoms of dysphoria increased in patients following naloxone, but not placebo administration. Naloxone-induced changes in symptoms of dysphoria were correlated with changes in opioid withdrawal as assessed by both subjective and objective measures. These findings suggest that dysphoric mood states in heroin addicts may be, in part, pharmacological sequelae of their drug dependence. Dysphoria due to opioid withdrawal may contribute to the initiation and maintenance of heroin use, and to the high rates of syndromal affective disorders reported in this population.