Fourteen 'carcinoid' tumours of the breast are described. They are separable into five with and nine without intracellular mucin. All the tumours are argyrophil, but none is argentaffin. Four tumours studied ultrastructurally contain dense-core granules. Argyrophil carcinomas represent the endocrine analogues of ductal carcinoma in situ, of invasive ductal carcinoma and probably of lobular carcinoma also. Current views vary between the one that the so-called carcinoid is a rare and totally distinct entity to the view, at the other extreme, that it is a very common variant of conventional breast cancer. On the basis of our findings, an intermediate view is justified: argyrophil carcinomas constitute about 5% of breast carcinomas and some varieties at least have non-argyrophil analogues. Factors influencing the prognosis in individual cases are discussed. Argyrophil carcinomas of the breast form a tumour spectrum with a wide range of morphological and histochemical appearances and a variable prognosis.