In situ lobular carcinoma is a preinvasive form of breast cancer which if inadequately treated will frequently progress to infiltrative cancer. A follow-up study of 50 patients with in situ lobular carcinoma, treated in most instances by local excision, reveals the following subsequent cumulative risk of infiltration: 8% after five years, 15% after ten years, 27% after 15 years, and 35% after 20 years. After 23 years, the cumulative risk is thought to exceed 50%. In addition, the cumulative risk of cancer developing in the breasts contralateral to those in which in situ lobular carcinoma was found was 10% after ten years, 15% after 15 years, and 25% after 20 years. The cumulative risk of contralateral breast cancer after 22 years appears to exceed 30%.