Background: Currently, topical immunotherapy with diphenylcyclopropenone (DCP) represents the most accepted therapeutic modality in the treatment of severe alopecia areata (AA). Objective: Efficacy, side effects and prognostic factors of DCP treatment in severe AA. Methods: Retrospective study of 68 patients with severe AA (>40% scalp hair loss), treated for at least 5 months at the Department of Dermatology, University Hospital of Zürich, between May 1989 and December 1996. Results: The overall response rate was 70.6%, complete remission was obtained in 30.9% and partial remission in 39.7%. Among the investigated prognostic factors for the outcome of DCP therapy, only the extent of AA at the time of initiation of treatment was found to be of significance. Conclusion: DCP treatment of severe AA is an effective, albeit symptomatic therapy with frequent side effects and a relatively high relapse rate. The response rate depends on the type of AA.