Current follow-up information is presented on a unique group of 46 patients first diagnosed as having lobular carcinoma in situ during the period 1940-1952 and observed from 4 to 27 years. Six patients each had radical mastectomy; the remaining 40 had no treatment after a diagnostic biopsy. Subsequent cancers developed in 33% (15/46) patients. The 15 patients developed 20 cancers; an incidence of 13% (6/46) bilaterality. Seventy-five percent (15/20) of the cancers were infiltrating. Two patients (4%) are dead from metastatic breast cancer; 2 others (4%) are living with metastases. Only half of the 20 cancers appeared in die first decade and the remaining half appeared up to 22 years after the original diagnosis of lobular carcinoma in situ. The evidence continues to accrue proving that lobular carcinoma in situ is a preinvasive form of breast cancer which should be treated by modified radical mastectomy.