Previous reports of skin cancer occurring in mycosis fungoides patients who have been treated with topical nitrogen mustard have been of carcinomas occurring predominantly on sun-exposed skin, Four of twenty-nine mycosis fungoides patients in our clinic treated with topical nitrogen mustard developed six skin cancers on sun-protected skin. Four lesions were squamous cell carcinomas and two were basal cell carcinomas. Few other risk factors for the development of skin cancer were present in our patients. These cases support the hypothesis that topical nitrogen mustard is a carcinogen or tumor promoter and emphasize the need for careful examination of the skin of patients treated with topical nitrogen mustard.