The aim of the study was to examine the relationship between therapeutic climate and the effectiveness of CBT treatment for sexual offenders in U.K. prisons. To this end a measure of group atmosphere was administered to members and leaders of 12 treatment groups running the same prison-based sexual offender treatment program. Treatment outcome was measured using a case-by-case methodology clinically significant change analysis--to identify the percentage of individuals within each group who had significantly changed on measures of pro-offending attitudes targeted in treatment. The results of the group process measure indicated leaders generally viewed groups more positively than members. Clear differences were also found between groups (ostensibly running the same treatment program) in terms of group climate. Analysis of members' process data indicated that there was a clear relationship between how cohesive the members reported the group to be and the extent to which freedom of action and expressions of feelings were encouraged in groups, as well as treatment outcome as measured by significant reductions in pro-offending attitudes. These results are discussed in terms of what they say about attitudes and goals of those involved in running treatment and how the effectiveness of treatment programs can be maximized.