During a period of ten years, 118 (32.9 per cent) instances of carcinoma were found in 359 specimens taken at biopsy for nonpalpable mammographic lesions. In recent years, the positive predictive value has increased from 68 per cent due to the development of magnification mammography and the use of a mammographic grid. Correlating mammographic and histopathologic data, the rate of malignant disease was 12.7 per cent for instances of a circumscribed or nodular mass, 32.4 per cent for clustered microcalcifications as the only suspect finding, 28.6 per cent when a mass with microcalcifications was present and 66.7 per cent when a stellate-shaped mass was found. Of 188 instances of carcinoma, 40 were noninvasive: 32 instances of ductal carcinoma (27.1 per cent) and eight of lobular carcinoma in situ (6.8 per cent). The possibility of frozen section diagnosis was studied retrospectively by comparison with the paraffin section reports. A correct diagnosis, whether benign or malignant, was achieved in 68 per cent. No frozen section examination was done in 17.3 per cent and the diagnosis was deferred to results of paraffin section in 12.2 per cent. False-negative results were encountered in seven patients (1.9 per cent) and false-positive results in two (0.6 per cent). Both of these patients had florid sclerosing adenosis. Although frozen section diagnosis is feasible in nonpalpable lesions of the breast, it is recommended that this method not be used in instances of pure microcalcifications and tiny solid masses of 5 millimeters or less.