Background: Highly variable results of topical diphenylcyclopropenone (DPCP) in the treatment of alopecia areata have been reported so far. Objective: The purposes of our study were to evaluate the efficacy and tolerability of DPCP in the treatment of chronic, extensive alopecia areata and to assess the long-term overall benefit of treatment. Methods: Fifty-six patients with chronic, extensive alopecia areata were enrolled in an open-label clinical trial. After sensitization with 2% DPCP, progressively higher concentrations beginning at 0.001% were applied weekly for 6 to 12 months to one side of the scalp. Results: Fifty-two of 56 patients completed therapy. Total regrowth of terminal hair was achieved in 25 of 52 patients (48%) at 6 months. The most frequent side effect was an eczematous reaction at the site of application. Notably, persistent response was observed in 60% of these patients after 6 to 18 months of follow-up (mean, 12 months). Conclusion: Topical DPCP treatment for alopecia areata is effective and well tolerated and provides prolonged therapeutic benefits. (J Am Acad Dermatol 2001;44:73-6.)