Because so‐called ‘carcinoid’ tumour of the breast has proven to be a difficult entity to define, we studiedin‐situcarcinoma as there were reasons to believe that this might help clarify the complex problems involved. We studied a consecutive series of 30 cases of ductal carcinomain‐situ(DCIS) by light microscopy and silver impregnation methods and identified a relatively common endocrine variant of DCIS. This variant was studied by immunocytochemical and ultrastructural methods, using conventional DCIS as a control. Endocrine DCIS is identified by its organoid pattern, stromal ‘inclusions’, festooned structure and a distinctive type of polypoid invagination. It is argyrophilic and rich in neuron‐specific enolase. Ultrastructurally it contains abundant dense core granules which are impregnated selectively by Grimelius' method. This tumour type frequently contains peptide hormones of the ACTH family. Three of seven cases contained cells reactive for ACTH and corticotropin‐like intermediate lobe peptide CLIP or their precursor, proopiomelanocortin. A fourth tumour contained neurotensin, recently identified in a variety of endocrine tumours. Argyrophil invasive carcinomas are a much more heterogeneous group of tumours than argyrophil DCIS and only a minority have an endocrine structure comparable to th