A prospective study on the progress of 356 women with unilateral carcinoma of the breast treated throughout at the Institut Gustave-Roussy, by a Halsted or Patey type radical mastectomy. Between 1-1-68 and 31-12-71 293 patients without any previous treatment were operated on, and 63 after irradiation according to the dosage-schedule then in force. In the first group no prognostic importance could be attached to the character of the tumour margin nor to its stroma. Two features thought to be bound up with survival on first analysis were not found to have significance when the other prognostic factors were kept constant and these are microscopic spread of tumour outside the nipple and transcapsular spread of tumour in the axillary nodes. Only four parameters were found to have a real prognostic value and these were the size of the tumour, its Bloom grading, microscopic spread to the nipple and the number of nodes invaded, with a threshold at three involved nodes. With regard to irradiated carcinomas, whose number were really too small for such an intense analysis, only two parameters were related to survival: Bloom grading and the number of axillary nodes invaded.