A 74-year-old woman with a right ovarian tumor had a high level of the serum tumor marker alpha-fetoprotein (AFP). After combination chemotherapy, the serum AFP level decreased significantly. Histopathologic study of surgically excised tumor tissue revealed a serous papillary cystadenocarcinoma. By the indirect immunoperoxidase technique, the AFP was stained in the cytoplasm of the differentiated tumor cells, showing papillary or tubular structures. Serial sections showed that the cells containing PAS-positive substances did not correspond to those producing AFP.