The ‘What Works’ literature has established that prison-based rehabilitation programs can reduce post-release re-offending rates amongst some offenders. Validated tools for measuring prison social climate have reliably identified regime factors that tend to make the prison experience less negative for prisoners. Experience in other human service areas would suggest that programs delivered in a positive prison social climate should be more effective than those delivered in a negative climate. However, the two lines of research exist in parallel without directly intersecting. This article examines research evidence that is laterally or tangentially relevant. The conclusion is that it would be perverse to structure penal administration policies around the view that a positive prison social climate cannot make any difference to re-offending rates. The evidence is that a good prison social climate would seem likely, other things being equal, to improve the outcomes achievable through proven ‘What Works’ rehabilitation programs. The research methodology to establish this correlation is complex. The article concludes by addressing these complexities and suggesting a viable methodology.