In situ canalicular carcinomas are defined as malignant, galactophoric epithelial cells, do not invade the basal membrane. The author saw 21 cases of in situ canalicular carcinomas which were treated surgically at the University Hospital of Rennes. It is possible from these cases to include certain themes: there is a pathology of an early carcinoma of the breast which will go on growing. It concerns patients in the menopausal age who 1 in 3 have a family history of neoplasia. Medical examination is important because in 1 out 2 cases leads to complimentary examinations which help to make the diagnosis. Mammography is essential because it leads to the diagnosis of a finding of suspicious microcalcifications as well as arranges an outline. The histology will indicate what the therapy should be. Comedocarcinomas (cellular proliferation with central necrosis) has to be considered as separate from other better differentiated types (solid, cribriform, papillary and clinging carcinomas). In fact comedocarcinomas often have a poor prognosis and the factors are: the size, the mitotic activity, the poor cellular differentiation, perigalactoriphic reaction, and stromal and multi centre micro-invasion. Treatment has to be adapted to features of this pathology. Mastectomy which used to be routine in these cases because of the high incidence of multiple centres, are to be reserved for cases with a poor prognosis. At present conservative treatment is being evaluated, but it seems logical to recommend it in non-invasive cancers with a good prognosis when a large lumpectomy followed up by radiotherapy to the whole breast can be used, particularly because this form of treatment is now given to invasive tumours. It is very important to follow-up these patients regularly and for a long time, clinically and with mammography if conservative treatment is carried out, in any case when the opposite breast is at risk. Conclusion. Now that treatments have become varied the improvement in prognosis will depend on early diagnosis. This means that mammography screening will proceed to an increasing number of non-invasive cancers of the breast that are discovered, and to a need to work out acceptable and effective treatment for these cancers.