This study explores the prevalence of criminal behaviour in patients addicted to drugs who are in treatment. A sample of 252 addicted patients (203 male and 49 female) who sought outpatient treatment at a specialized centre was assessed. Information on criminal behaviours, socio-demographic factors, consumption factors (assessed by the EuropAsi), psychopathological factors (assessed by SCL-90-R) and personality variables (assessed by MCMI-II) was collected. Patients presenting criminal behaviour were compared with those who were not associated with crime for all the variables studied. The rate of drug-addicted patients with criminal behaviour in this sample was 60.3% (n = 150), and it was mainly related to traffic offenses, followed by drug dealing offenses. Significant differences were observed between patients with and without criminal behaviour. Patients with criminal problems were mostly men and single. Moreover, they were more likely to report poly-consumption. Furthermore, significant differences were observed on several variables: EuropAsi, SCL-90-R and MCMI-II. According to these results, patients with associated criminal behaviour presented a more severe addiction problem. The implications of these findings for clinical practice and future research are discussed.