Aerial parts of AGERATINA PICHINCHENSIS have been used, in Mexican traditional medicine, as a remedy for the treatment of skin mycosis. Onychomycosis, also known as tinea of the nails or tinea unguium, constitutes an infection of the nails produced by dermatophytes. Clinically, onychomycosis is manifested by changes on the color, texture and thickness of the nail. The agent most frequently found in this disease is TRICOPHYTON RUBRUM. The present study evaluated the therapeutic effectiveness and tolerability of topical administration of A. PICHINCHENSIS extract on the nails of patients with the clinical and mycological diagnosis of onychomycosis. A phytopharmaceutical formulation was developed in a lacquer solution containing the standardized (encecalin) extract of A. PICHINCHENSIS. A similar lacquer solution containing 8 % ciclopirox was used as control. Treatments were assigned randomly and administered topically for 6 months. Ninety six patients concluded the study (49 in the experimental group and 47 in the control); 71.1 % of patients from the experimental and 80.9 % from the control group showed therapeutic effectiveness, while 59.1 % and 63.8 % from the experimental and control group, respectively, achieved mycological effectiveness. Therapeutic success was observed in 55.1 and 63.8 %, respectively. No patient exhibited intense side effects. Statistical analysis demonstrated no differences between treatments.