The Second Department of Surgery at Tokyo Women's Medical College used ultrasonography to evaluate Forty cases of nonpalpable breast tumors between January 1988 and April 1989. These did not include obvious cysts and mastopathies. Ultrasonography makes it possible to locate dilated lactiferous ducts, microcalcifications and small solid tumors among nonpalpable breast lesions rapidly and accurately. The department classified the ultrasonographic findings into 2 groups. Group 1 was composed of 25 patients with dilated lactiferous ducts. Based on the ultrasonographic pattern of the ducts, they were then broken down into Types I and II, with Subtypes a, b and c. Group 2 consisted of 15 patients; 6 with microcalcifications and 9 with small solid tumors. Based on the ultrasonographic patterns, the microcalcifications were broken down into Types A, B and C; the small solid tumors were broken into Types 1 and 2. The department then collated the ultrasonographic classifications with the histopathological diagnoses. As a result, Types I, A, B and 1 were classified as benign; Types II, C and 2 as malignant. Using ultrasonography, the department diagnosed the locations of abnormal lesions in 100% of the 40 cases. Using mammography, 30.0% could be diagnosed; and with galactography, 83.3%. The diagnostic sensitivities of ultrasonography, mammography, galactography and cytology were 100%, 28.6%, 50.0%, and 50.0%, respectively; the specificities of each method were 76.9%, 7.7%, 50.0%, and 66.7%, respectively. The department concluded that ultrasonography is an indispensable diagnostic tool, not only in locating nonpalpable breast tumors, but also in differentiating breast cancers from benign tumors.