In 46 patients with recurrent breast cancer, the chronological change of pathomorphologic findings and its relationship with the clinical courses were studied with the comparison between the materials obtained at the time of operations and autopsies. Histologically, 24 cases (52.2%) showed undifferentiated change with time, and 22 cases (47.8%), no change. There were no cases with differentiation. The cases with undifferentiated change progressed acutely with no response to treatment, resulting in poor prognosis. Some of these cases showed metastatic behavior similar to leukemia, malignant lymphoma or small cell carcinoma of lung. It was suggested that the chronological and histological change was to some extent related to the clinical course.