Two groups of drug users on an inner-city inpatient drug detoxification unit were studied: 42 heroin addicts and 47 cocaine addicts. The two groups were compared on personality disorder diagnoses, personality traits, and demographic variables. Cocaine and heroin addicts scored similarly on: 1) number and kind of personality disorder diagnoses, with the exception of antisocial personality; 2) all personality traits measured; 3) positive and negative temperament; 4) description of self-concepts; and 5) positivity and negativity of self-concepts. It was also found that heroin addicts showed significantly higher levels of social deviance than the cocaine group, with significantly more antisocial personality disorder diagnoses, higher levels of social deviance, and lower scores on a Propriety Scale. Character pathology was more heterogeneous among cocaine users. Heroin addicts had used significantly longer and showed less educational and occupational achievement than the cocaine group. Our results also suggest that personality psychopathology in drug addicts is associated with lower self-esteem, more negative self-valuation, and longer duration of use.