Histological sections of the primary tumour and of homolateral axillary lymph nodes from 500 women with operable invasive breast cancer have been examined. The tumours have been graded and the degree of round cell infiltration assessed. These features, together with clinical palpability and pathological involvement of axillary nodes, have been related to survival. It was found that prognosis was worse in patients with a high grade tumour and in those with histological evidence of axillary node spread. Round cell infiltration of the primary tumour did not confer improved survival. The clinical state of the axillary nodes was associated with prognosis in so far that palpable nodes were twice as commonly the seat of metastatic spread as were impalpable nodes.