Lobular carcinoma in situ (LCIS) is not only a relative newcomer among breast lesions, but in its short span of 50 years it has gradually evolved from a rare form of breast cancer to being merely a marker of increased risk. This change has not been without controversy which persists to the present day, although there is now general agreement on the natural history of the disease. The present report represents an update on current thinking about LCIS as well as a review of the limited number of studies dealing with the natural history of the lesion when treated by biopsy alone. Invasive cancer will develop in approximately 20-25% of women with LCIS provided there is sufficient follow-up after biopsy. Precise estimates are not possible since LCIS is an asymptomatic lesion that never makes a mass or reveals itself on mammography. It is found only by biopsy and thus the population being followed is a selected one. Every study has shown that when invasive cancer develops, it is just as likely to appear in the contralateral as in the biopsied breast, and invasive ductal cancers are more common than lobular. Clearly, the small round cells with pale cytoplasm that characterize LCIS do not go on to invasion in the usual patient; rather they serve to identify women who are more likely to develop breast cancer. Such patients represent a clearly defined group at increased risk, and for that reason are ideal candidates for chemoprevention. If tamoxifen or some other agent proves to be effective, the remaining arguments favoring mastectomy for LCIS will finally disappear.