--- - |2+ Abstract Background Interventions for offenders with mental illness have tended to be confined to treatment of illness, with the expectation that symptom reduction will be accompanied by reduced criminal recidivism, but recent evidence suggests that other treatment targets may be more effective against recidivism. Aim The aim of this study was to examine the effect of a cognitive skills programme (Reasoning and Rehabilitation 2: Short Version for Adults [R&R2]) among offenders with mental illness. Our first hypothesis was that participation in this programme would result in significantly greater improvement in antisocial attitudes than among similar prisoners who did not participate; both groups received “treatment as usual” (TAU). Our second hypothesis was that those receiving R&R2 would show less post‐treatment violent or general recidivism than those receiving TAU alone. Method Incarcerated offenders with serious mental illness (N = 101) were randomly assigned to R&R2 or TAU alone. Criminal attitudes and mental state were examined before and after treatment. Violent and non‐violent recidivism was measured, on average, 18 months after release. Results In both intervention and TAU alone trial arms, there were significant pre‐ to post‐treatment changes in criminal attitudes and symptoms or signs of mental disorder. There was no difference between groups in these respects. These pre/post changes were not associated with reductions in recidivism over time, whether or not controlling for baseline risk. Conclusions Although there was no demonstrable advantage of R&R2 over TAU alone, non‐significant trends towards lower violent recidivism in the R&R2 group and general recidivism in the TAU group suggest that it may be worth repeating the trial in a larger sample with more differentiated control groups. - Criminal Behaviour and Mental Health, Volume 28, Issue 4, Page 369-382, August 2018.