Twenty-three patients were enrolled in a randomized, double-blind trial of terbinafine 1% cream versus its vehicle (placebo) in the treatment of tinea cruris. One patient had a negative initial culture and was excluded, and two patients were dropouts, one because of poor study compliance (terbinafine) and one because of an adverse event (placebo). Twenty patients were examined for efficacy of treatment (9 terbinafine-treated, 11 placebo-treated). Both groups were similar in age, sex, duration of disease, prior therapy, size and location of lesion, infecting organism, and predisposing factors. Terbinafine 1% cream was more effective than vehicle cream in the reduction of the signs and symptoms of tinea cruris. In addition, there was a higher conversion rate to negative culture and normal microscopy findings in the terbinafine-treated group. Clinical results combined with evaluation of mycologie tests at end of therapy showed terbinafine to be a rapid and significantly more effective treatment for tinea cruris than placebo (78% vs 18% cure rate, respectively). Follow-up cure rates confirmed these findings (89% and 18%, respectively). No significant adverse events occurred during terbinafine treatment. © 1990, American Academy of Dermatology, Inc.. All rights reserved.