The purpose of the current study was to examine to what extent risk factors and treatment readiness were related to engagement (i.e., participation and completion) in prison-based rehabilitation programs. The sample consisted of the total 6-month inflow of male detainees in the Netherlands who were assigned a candidate for a prison-based rehabilitation program (N = 638). Logistic regression models showed that treatment readiness partially explained treatment program completion. Offenders who were ready for treatment were more than two times as likely to complete treatment programs, compared with offenders who were not. Risk factors (such as drug or alcohol misuse) did, with a few exceptions, not correlate with treatment participation or treatment completion. Outcomes pointed to the importance of treatment readiness and showed the significance of enhancing treatment readiness among offenders who are eligible for correctional treatment programs. Results were discussed in light of study limitations and suggestions for future research.