Two matched therapy groups were studied for nine months in terms of process and outcome. In the "experimental" group, the therapist was trained to use techniques of social reinforcement to facilitate the development of intermember cohesiveness—also termed intimacy, solidarity, or affection. In the comparison group, the therapist, who was matched by personality with the "experimental" therapist, used a more conventional, intuitive, groupcentered approach in dealing with the group. Results indicate that the patients in the "experimental" group showed significantly more cohesiveness, greater personality change in predicted directions, and earlier symptomatic improvement than those in the "comparison" group. The potency of the therapist in shaping and modifying group dynamic behaviors is established by the data for both groups. The findings support the utility of a reinforcement or learning approach to the understanding and practice of group therapy.