Among 412 female patients with fibroadenoma 30 (7.1%) were shown to have considerable epithelial proliferations, 7 (1.7%) carcinoma in situ (n = 4) or carcinoma (n = 3). All patients with carcinoma or carcinoma in situ in the fibroadenoma were older than 40 years of age. Differentiation of coarse or fine granulation of microcalcifications did not improve the accuracy of mammography. In 32% of fibroadenomas with epithelial proliferations similar changes were also found in the adjacent area of the breast whereas only in 8.8% of fibroadenomas without proliferative changes could epithelial proliferation be observed in adjoining areas. With increasing age the risk of carcinomatous degeneration in fibroadenomas rises to 17%. In multiple fibroadenomas only one of them may show carcinomatous changes whereas the others may appear unremarkable. This necessitates excision of all such tumours. In the same way as carcinomas fibroadenomas are most frequently found in the upper outer quadrant of the breast. Total resection including a surrounding layer of healthy tissue remains the treatment of choice.