This study explored the prevalence of violent and/or criminal behaviors in drug-addicted patients. A sample of 252 drug-addicted patients who sought treatment was assessed. Information was collected on violent behaviors, criminal acts, socio-demographic factors, consumption factors, psychopathological factors, and personality variables. The sample was divided into 4 groups according to the presence of violence and/or criminal behaviors. There were significant differences between the groups on some variables. In general, patients associated with both violence and criminal behaviors showed a greater severity in drug consumption and maladjustment variables, as well as a higher rate of treatment dropout and re-entry.