The effectiveness of treatments for substance use disorders is strongly related to retention, since early dropout from treatment is associated with greater likelihood of relapse. The purpose of this prospective, ex post facto study is to analyze the effect of individual variables on retention in a treatment program carried out in a prison drug-free unit. The Addiction Severity Index, motivation and personality profile of fifty inmates were assessed on entry to the prison. Inmates were monitored for a year to identify length of stay. Results: Motivation variables at intake play a vital role in the prediction of retention in a prison drug-free unit; scores on the Aggressive-Sadistic and Narcissistic scales are also strong predictors of treatment retention.