27 patients suffering from either extensive alopecia areata (n = 5) or alopecia totalis (n = 22) were treated topically with diphencyprone, a new potent contact allergen. The duration of treatment ranged from 4 to 17 months. Unilateral induction of hair growth after unilateral treatment was observed in 23 patients. A continuous response after continuous treatment has been observed so far in 18 of these patients. Thus, diphencyprone was found to be as effective as DNCB or squaric acid dibutylester in the treatment of alopecia areata. Unlike DNCB, diphencyprone is not mutagenic in the Ames test. Compared with squaric acid dibutylester, diphencyprone is more stable and thus more suitable for storage when dissolved in acetone. Further investigative evaluation of diphencyprone may show whether this drug is suitable for a more general use in the treatment of severe forms of alopecia areata.