172 patients who had had mastectomy for breast cancer were treated by repeated adjuvant chemotherapy, either with phenylalanine mustard (P.A.M.) or a combination of cyclophosphamide, 5-fluorouracil, and prednisone (C.F.P.) with and without radiotherapy. Tumours recurred significantly more frequently and mortality tended to be higher in P.A.M.-treated patients than in patients on other treatment. The interval between surgery and disease recurrence was significantly shorter for P.A.M.-treated premenopausal but not postmenopausal patients than for patients of equivalent menstrual status treated with C.F.P. with or without radiation. The associations in premenopausal patients between the mode of treatment and both survival and the disease-free interval were significant before and after adjustment for variations between the treatment groups in the number of involved lymph nodes and the size of the primary tumour.