OBJECTIVE Based on an epidemiologic field survey of community households in Puerto Rico, this study estimates the frequency of illicit drug use and clinically defined drug abuse and/or dependence syndromes. Results are compared with those from surveys on the United States mainland. Suspected risk factors are studied as well, with a special focus on childhood misbehavior. METHODS Trained lay interviewers administered a Spanish Diagnostic Interview Schedule to 912 respondents aged 17 to 68 years who were selected by multistage probability sampling of island households. RESULTS An estimated 8.2% of the population had a history of illicit drug use and 1.2% qualified for a standardized lifetime diagnosis of drug abuse, dependence, or both. An estimated 18.4% of the male drug users and 7.7% of the female drug users met criteria for drug abuse and/or dependence. A history of drug use was related to the diagnoses of alcohol abuse and/or dependence and antisocial personality, but few persons who had used illicit drugs at least once in their lifetime reported a history of receiving treatment for alcohol, drug, or mental health problems. CONCLUSIONS The data were consistent with a suspected association between level of childhood misbehavior and occurrence of illicit drug use, even after statistical control for potentially confounding variables.