Several recent reports have described an increase in the incidence of preinvasive carcinoma of the breast. To determine the incidence of preinvasive breast cancer at our institution, the results of 469 consecutive outpatient breast biopsies performed at Walter Reed Army Medical Center between July 1, 1985, and January 1, 1987, were reviewed. During this time period, 256 biopsies were performed on palpable masses, and 213 needle localization biopsies were performed on mammographically suspicious lesions. The overall incidence of cancer was 15.4%. Needle localization biopsies yielded a diagnosis of cancer in 17.8% of cases, as did 13.3% of biopsies performed on palpable masses. Eight of 38 (21.1%) carcinomas identified by mammography were preinvasive at the time of diagnosis. Only one of 34 (2.9%) cancers identified because of a breast mass was preinvasive. We conclude that screening mammography is an invaluable tool for the detection of preinvasive carcinoma of the breast and that the increased use of mammography will result in an increase in the incidence of detection of this lesion.