A sample of 110 drug-involved offenders from two prisons was assessed regarding drug and alcohol consumption, problem awareness, ambivalence and treatment readiness. Of these, 56% reported hazardous alcohol consumption and 53% highly problematic drug use. Highly problematic users reported more problem awareness and more cognitive dissonance regarding change. Treatment readiness was rated higher by problematic users as compared to users reporting fewer problems. The influence of prison environment on ambivalence needs to be studied longitudinally after imprisonment. Identification of drug-involved prisoners should be done systematically, including assessment of alcohol consumption. Opportunities for substance misuse treatment in prisons should be increased.