Mammary hamartomas are macroscopically well-delineated tumours composed of a variable mixture of epithelial elements, fat and fibrous tissue. Such lesions are an under-recognized entity and, as they can be visualized by mammography, may be seen more frequently with the advent of the UK National Breast Screening Programme. The clinical and pathological features of 35 cases of mammary hamartoma seen at the Imperial Cancer Research Fund Clinical Oncology Unit at Guy's Hospital between 1979 and 1990 have been reviewed. Hormone receptor analysis on nine cases gave high progesterone with low oestrogen levels, probably reflecting their premenopausal status. Immunohistochemistry showed that the positive receptor staining was confined to the epithelial elements. In 25 cases pseudo-angiomatous hyperplasia was evident in the stroma of the lesion. The importance of distinguishing the inter-anastomosing stromal spaces seen in the latter condition from low-grade angiosarcoma is emphasized; the relationship between pseudo-angiomatous hyperplasia and mammary hamartoma is discussed; and the possibility that the former represents a permanently dilated form of the lymphatic labyrinth suggested.