To evaluate the relative effectiveness of the treatment of tinea pedis, we performed a double-blind study comparing undecylenic acid ointment, tolnaftate cream, and a placebo. Ninety patients with clinically and mycologically proven tinea pedis took part in this clinical trial. Our results showed both agents to be superior to the placebo, but there was no significant difference in effectiveness between the two active agents. We observed no side effects with the preparations used.