ABSTRACTBackground: With notable exceptions, few studies have looked critically at the role and effects of factors other than individual or programmatic differences that contribute to the climate within substance abuse treatment programs. De Leon's work on the therapeutic community, however, indicates that factors beyond the individual and program can contribute to the overall functioning of similarly situated communities. Objectives: In this study, we introduce and examine the concept of the “treatment group,” the level of aggregation between the individual who participates in treatment and the organization that provides it. The treatment group refers to the social context and dynamics that operate among treatment participants, and we sought to study differences in treatment climate across 12 prison-based treatment groups within a single prison. Methods: Using data from 604 participants who were assigned at random to one of the 12 groups, we analyzed differences on seven treatment climate indicators from one month and six months into the program across the treatment groups. Results: No differences were found among the treatment groups at after one month; however, after 6 months, significant differences emerged across the groups on three treatment climate variables: program structure, counselor rapport, and counselor competence. Conclusions/Importance: This study is among the first of its kind to conceptualize and examine treatment climate across otherwise similar groups and find that differences emerge on several indicators of climate. As such, we argue that the treatment group may be important to consider in both the delivery and evaluation of substance abuse treatment.