Mucoid carcinoma of the breast is not a single homogeneous entity. It comprises two main variants separable on structural and cytological grounds and a smaller transitional type. The salient pathological differences are detailed. One variant is usually arygrophilic and contains dense core granules of the type seen in endocrine tumours. Both endocrine and amphicrine cells have been identified in this subtype. The relationship of this endocrine variant of mucoid carcinoma to other argyrophil carcinomas (so-called ‘carcinoids’) of the breast is discussed.