This article presents a study of a number of socio-demographic factors that predict treatment dropout from a residential substance use disorder programme in Spain (Proyecto Hombre Therapeutic Community). A sample of 430 patients (173 who completed treatment and 257 who dropped out of treatment) was assessed at entry to the therapeutic community programme in order to collect information on socio-demographic characteristics of the participants. According to the results obtained, factors that predicted treatment dropout included younger age, male gender and previous treatment history in the Proyecto Hombre therapeutic community. Patients with these characteristics were at significantly greater risk of dropping out of treatment. The implications of these results for further research and clinical practice are commented upon.