Authors review 43 cases of occult breast cancer selected from 6396 patients operated on for breast cancer in the National Institute of Oncology, Budapest over a 20-year period (1972-1991). In 20 patients mastectomy and axillary block dissection, in 9 upper-outer quadrant resection of the breast with axillary block dissection, in 12 only axillary block dissection, and in 2 axillary biopsy were carried out. The primary tumor was localized only in 11 patients during the first operation either by macroscopic or by microscopic examination. In 12 patients' breast the primary tumor became detectable only later. Out of the 20 mastectomy specimens, 2 primary tumors could not be identified even with the most thoroughful examination. Out of the breast conservation group in 4 patients, the primary tumor remained occult until recently, and in the axillary block dissection group (12 patients) the site of the primary cancer could be identified only in 2 patients. The authors deal with diagnostic difficulties and the dilemmas of the surgical indications and treatment.