Follow-up information was obtained on 199 women with breast biopsy specimens containing intraductal epithelial proliferation. The proliferations were divided into regular or ordinary intraductal hyperplasia (IDH) (117 cases) and atypical intraductal hyperplasia (AIDH) (82 cases). The average length of follow-up was 14 years for the patients with IDH and 12.4 years for the patients with AIDH. Of the 117 patients with ordinary IDH, carcinoma subsequently developed in six (5%); three of these were invasive carcinomas (2.6%). All three invasive carcinomas were in the ipsilateral breast, but of the three intraductal carcinomas (IDCa), two were in the contralateral breast. Of the 82 patients with AIDH, invasive carcinoma subsequently developed in eight (9.8%); six of these were located in the ipsilateral breast and two in the contralateral breast. One of these six patients died of disseminated carcinoma. The average interval to the subsequent carcinoma (intraductal and invasive carcinoma) was about the same in the two groups (8.3 years for AIDH and 8.8 years for IDH lacking atypia). When considering only subsequent invasive carcinomas, the interval was 8.3 years for the AIDH and 14.3 years for the IDH lacking atypia. Of the 14 patients with IDH and a family history of breast carcinoma, invasive carcinoma subsequently developed in one (7%) as compared with two (2%) of the 91 with a negative family history. Among patients with AIDH, invasive carcinoma subsequently developed in two of the 13 (15%) of those with a family history of breast carcinoma as compared with one of 57 (1.8%) of the women with a negative family history. The presence of atypia in epithelial hyperplasia is a significant factor in increasing the likelihood of the development of subsequent invasive carcinoma (P = 0.03; two-tailed test). Of women with AIDH, invasive carcinoma subsequently developed in 17% of those with sclerosing adenosis (SA) as compared with 4.2% of those without it. Therefore, SA may be a contributing factor to increased risk. A positive family history also appears to increase the likelihood of the subsequent development of invasive carcinoma, particularly in patients with AIDH.