Vulnerability to drug abuse and psychopathology was explored among siblings of 201 opioid-addicted probands. Disorders were evaluated based on family histories among 476 siblings; a subset of 133 siblings was also directly interviewed. Results indicated that a) siblings of opiate addicts had substantially higher rates of several disorders in comparison with rates in the community; b) as compared with parents of addicts, siblings had elevated rates of substance abuse and antisocial personality; c) the presence of a major psychiatric disorder significantly increased the risk of developing substance abuse among siblings; and d) psychopathology appeared to precede drug abuse in terms of age of onset in this group. Results obtained with the interviewed sample of siblings were replicated in the overall group. Findings of the study are discussed in terms of implications for the classification and treatment of substance abuse.