Forty-two patients with spider phobia, fulfilling the DSM-III-R criteria for simple phobia, were assessed with behavioral, physiological and self-report measures. They were randomly assigned to two group treatment conditions: (1) small groups of three to four patients, and (2) large groups of seven to eight patients. They received one 3-hr session consisting of exposure and modeling. The results showed that both conditions yielded significant improvements on almost all measures, and these were maintained or furthered at the 1-yr follow-up. With one exception, there was no difference between the conditions, but on most measures there was a trend for the small group condition to yield better effects. The proportions of clinically significantly improved patients at post-treatment were 82% in the small group and 70% in the large group. At follow-up the corresponding figures were 95% and 75%, respectively. The conclusion that can be drawn is that one-session group treatment is a feasible alternative to individual treatment, yielding almost as good effects.