We report 3 cases of erythema multiforme following topical application of diphenylcyclopropenone (DCP) for the treatment of alopecia areata. This eruption represents a rare side effect which could be controlled with corticosteroids given both systemically and topically. When this unusual reaction occurred, treatment with DCP was stopped. In one of the patients, subsequent topical immunotherapy with squaric acid dibutylester was not complicated by this side effect and resulted in complete hair regrowth.