A detailed review was made of the histopathology of 227 tumours taken from 261 women under 45 years of age with breast cancer. The tumours were classified as follows: whether oral contraceptives had been used at any time; and whether oral contraceptives had been used before first term pregnancy. All analyses were adjusted for the effects of age. Overall, 201 (88.5%) of tumours were infiltrating ductal in type, 19 (8.4%) were infiltrating lobular, and seven (3.1%) were in situ ductal. Of the infiltrating ductal tumours, 28 (14%) were grade I 88 (44%) grade II, and 84 (42%) grade III. Various other tumour characteristics were also examined (per cent of carcinoma in situ, lymphatic permeation, necrosis, lymphoplasmocytic reaction and tumour edge). None of the histopathological features assessed showed any significant association with oral contraceptive use. Some characteristics of the areas of breast tissue adjacent to the tumours were also studied. The degree of cyst formation was considerably less pronounced in those using oral contraceptives before first term pregnancy than in those not doing so.