Lobular carcinoma in situ of the breast is a well defined pathologic entity which is found in about 2.5 per cent of all specimens of the breast taken for biopsy and most commonly occurs in premenopausal females. Its diagnosis is virtually always incidental due to the absence of any clinical indication of its presence. This lesion carries a significant risk for development of subsequent invasive carcinoma which applies equally to both breasts and which appears to increase with time. The appropriate treatment of this disease remains a controversial issue. Various aspects of its epidemiology, pathology and natural history which have an important bearing on the therapeutic decision as well as the many treatment options available are analyzed herein. There is certainly a perception that lobular carcinoma in situ represent the early form of a malignant process which can be cured or prevented if appropriately treated at this stage. At the very least, an understanding of this lesion holds the potential for broadening our understanding of the physiologic basis of carcinoma of the breast as a whole.