In the absence of gross deformity of the nipple, such as its retraction or Paget's disease, histological examination of this area is often neglected, or at best confined to a cursory look at a single sagittal section. The inadequacy of this approach is illustrated by this study of 33 consecutive cases of carcinoma of the breast treated with mastectomy. Multiple transverse sections showed tumour in 19 nipples (58%) involving one or more levels. Of these, 17 showed non-invasive tumour, either ductal or lobular type. Invasive tumour was seen in only two nipples, one of which was metastatic extension from the underlying breast tumour. Paget's cells were seen in two cases. The most significant finding was the eccentric location of tumour in 14 nipples. A single central sagittal section would have detected only five cases involving the centrally situated duct. An inexplicable finding was a preponderance of right nipple with tumour. No statistically significant correlation between nipple involvement and the size, location, multicentricity, type of tumour in the breast and metastases in axillary lymph nodes could be found. It became evident from this study that malignant changes in the nipple occur more commonly than is realised, and that it is also one of the sites of multicentric origin of the tumour. This factor will have to be taken into account in planning conservative therapeutic programmes.