The prognostic significance of 16 factors was investigated in a series of 311 women with unilateral, invasive breast carcinoma without distant metastases (M0) and treated with mastectomy. The series consisted of 93% of such cases diagnosed histologically in the city of Turku, Finland, from 1980 to 1984. Mitotic count, histological and nuclear grades, extent of tumor necrosis, axillary nodal status (pN), tumor size (pT), estrogen and progesterone receptor (PR) contents, and S-phase fraction were the most powerful single factors (p = 0.0001 or less). Axillary node negative cancers with no or only spotty tumor necrosis (92% of all pN0 cases) were associated with a 96% 5-year survival rate corrected for intercurrent causes. Among the axillary node positive cases the combination of high PR content (greater than 60 fmol/mg protein) and a low mitotic count could identify a subgroup with a 96% 5-year corrected survival rate (25% of all pN+ cases). It is concluded that both pN0 and pN+ breast cancer contain identifiable subgroups with greatly different prognosis.