Marathon groups offer individuals an opportunity to engage in intensified, authentic personal encounter with each other in a small group setting, usually with 10-15 persons in a group. This is a report of tentative findings at the Student Life Center, University of Colorado. There were three matched groups, each with nine sophomores. The first (on-going) group met one and one-half hours each week for eight weeks. The second group met 12 hours over a weekend (marathon group). The third group served as a control. The two co-leaders functioned in both encounter groups. Both groups went through similar dynamic phases, and were characterized by honest and satisfying patterns of relating, self-exploration, and self-disclosure. The marathon group was felt by members to be the more rewarding experience. Data (reflected by a measure of increased feeling of internal control over the pattern of one's reinforcements) suggest that on-going group members became slightly more internalizing, while the marathon group became somewhat less internalizing. Future research, using the marathon in a variety of ways, will build in questions dealing with the individual within the dynamic interpersonal encounter. (KP)