In situ lobular carcinoma of the breast is relatively uncommon but represents the earliest of the early cancers which occur in the mammary gland. Because of obscure physical findings, it is rarely diagnosed clinically and frequently missed in gross and quick frozen section examinations. A considerable majority of the breasts show a multicentric origin of in situ lobular cancer and there is a high risk of leaving residual disease after a generous local excision. The frequency and the time at which a breakthrough will occur with resulting infiltration are not known but it seems to be a matter of years rather than months. Only one of the 31 patients listed as having been treated by local excisions in this series is known to have subsequently developed infiltrating cancer in the same breast. Most of these cases, however, were reported as showing only a single or very few microscopic foci of in situ lobular cancer in the multiple fixed sections from a local excision. Quite clearly we need to know more about the natural course of this disease but, unfortunately, unlike the cervix, the breast is not easily subjected to frequent cytologic examinations. The high incidence of in situ cancer and/or infiltrating cancer in the opposite breast has been stressed. Finally, I would like to emphasize that those of us on the Breast Service at the Memorial Hospital believe that the longterm infiltrative potentialities of in situ lobular cancer should not be underestimated and that in most patients the best treatment is a modified radical mastectomy. All of the patients so treated have shown no clinical evidence of recurrent disease.