A double-blind study involving 62 male inpatients of a hospital which handles long-term cases was made to assess the efficacy of miconazole nitrate (1-[2,4-dichloro-β-(2,4-dichlorobenzyloxy) phenethyl] imidazole nitrate) in the treatment of chronic tinea pedis. Two groups of 31 patients, matched for severity of lesions, were assigned to treatment with either miconazole nitrate or the (control) vehicle alone. Miconazole nitrate produced highly significant (P < 0.001) clinical improvement which was maintained at the same level throughout the six-week follow-up. No side effects were ever observed. Trichophyton interdigitale was isolated on 18 occasions before treatment, T mentagrophytes on four, T rubrum on five, T mentagrophytes varietas granulosum on three, and T megninii on one. No dermatophytes were observed after miconazole nitrate treatment.