In this study breast tissue from 114 patients has been examined ultrastructurally for dense core granules (DCG). The tissue included examples of normal 'resting', pregnant and lactating breast plus various benign and malignant lesions. DCG were observed in low numbers in the apical cytoplasm in a proportion of the examples of 'resting' and pregnant breast tissue but were absent in the lactating patients. The incidence appeared to relate to hormonal changes. They were present in 50 per cent of the benign lesions examined. DCG were also observed in a high proportion of the ductal, lobular and tubular carcinomas examined and were associated with luminal differentiation. In the mucoid carcinomas over half the tumours possessed some DCG with large numbers of DCG present within certain of the malignant cells in two cases. It is possible that the granules could be related to mucin secretion. Therefore, in normal, benign and malignant (with the exception of mucoid carcinoma) breast tissue the presence of DCG would appear to be related to hormonal changes and represent prelactational differentiation rather than providing evidence of neuroendocrine differentiation. We emphasize the need for a comprehensive knowledge of the normal morphological variations within a tissue before attempting to interpret its tumours.