Twenty-eight women with ductal carcinoma in situ (DCIS) of the breast treated by biopsy only were identified in a histologic review of 11,760 biopsies performed between 1950 and 1968. Seven of the 25 women followed for more than three years developed invasive breast carcinoma, all in the same breast with a previously detected DCIS. Average follow-up interval for the 18 women not developing invasive carcinoma was 16 years. The invasive carcinomas presented clinically from three to ten years (average, 6.1) after the biopsies demonstrating DCIS. Four women with invasive carcinoma developed distant metastases following mastectomy. This study suggests that 28% of women treated with biopsy only for DCIS presenting as an incidental histologic finding will develop invasive carcinoma in a follow-up period of approximately 15 years.