A follow-up period averaging 21.6 years was obtained for patients with low-grade papillary intraductal carcinoma initially treated only by biopsy between 1940 and 1950. Subsequent carcinoma was diagnosed in the same breast in seven of the ten patients after an average interval of 9.7 years. Six of the seven subsequent carcinomas were invasive. Two of the patients died of metastatic carcinoma and two were known to be alive with metastases when last contacted. Three patients were without carcinoma following mastectomy. When these results were combined with the few reports available in the literature, it appeared that at least 39% of patients with intraductal carcinoma treated by biopsy alone subsequently had clinically evident carcinoma, invariably in the same breat, with an average latent period of about ten years. This was undoubtedly a result of the multicentric nature of the disease in many patients.