Presented is a preliminary study into the effectiveness of a cognitive skills programme, Thinking Minds, conducted with an adult male forensic psychiatric population (n = 27; 18 treatment, nine waiting list controls). It also addresses the approach to evaluating treatment effectiveness by capturing both group and individual effects. All participants were given a series of measures, to assess domains where treatment effect was thought likely to occur. This included impulsivity, coping, emotional control and self-esteem. It was predicted the treatment group would evidence positive change following the intervention, with no change in waiting list controls. Results indicated partial acceptance of the group effect prediction, with the waiting list control demonstrating no group change across time and the treatment group demonstrating improvement in rational and detached coping and in the social component of self-esteem. The individual change results demonstrated a mixed picture. It confirmed improvement in adaptive coping and social self-esteem for the treatment group but widened positive effects to cover aggression control. It also indicated evidence of deterioration on outcome measures. Deterioration was noted across all measures for the control group, suggesting that a degree of deterioration may be a naturally occurring process on self-report measures, regardless of intervention. This is an issue that future evaluations need to reflect on and accommodate. Results are discussed with regards to how the findings can begin to influence our approach to treatment evaluation