In mastectomy specimens, the primary foci of occult breast carcinoma were usually examinated by routine histopathological method, but the result was not satisfactory. The detecting rates of primary foci were 50%-56% in China and 45%-75% in some other countries. In this study, whole organ subserial section was performed in 20 cases of occult breast cancer from April, 1988 to February, 1994. Primary foci were found in 16 cases (80%) by microscopic examination. Diameters of 10 foci were less than 1.0 cm with the smallest as 0.3 x 0.1x 0.1 cm. In addition, occult multiple foci were detected in 5 cases (31.25%), which would be very difficult to be found by routine histopathological examination. ER, PgR and monoclone M₄G₃ assays were performed for positive lymph nodes to confirm the primary loci to be breast cancer. The possible causes for the failure of detection of the primary foci by whole organ section are discussed.