A patient with slowly progressive tumor-stage mycosis fungoides (MF), without apparent extracutaneous involvement, received topical immunotherapy with mechlortethamine hydrochloride as the immunogen. A complete remission was achieved after two months of treatment, and the patient has remained free of detectable disease without further treatment for more than seven years. This case is also unusual in that, before treatment, a temporary exacerbation of MF had occurred during three pregnancies. These observations suggest that the natural resistance against the progression of MF in this patient was enhanced by immunotherapy.