The authors studied data of combination assays of tumor markers, because simultaneous elevation of different types of tumor markers in the serum was puzzling. They interpreted such phenomena regarding cancer antigen 125, carcinoembryonic antigen, and carbohydrate determinant 19-9 in ovarian tumors. The tissue expression of the antigens was compared with preoperative serum levels. Several different factors were found to cause the simultaneous elevation of two or three of these markers in the serum. Furthermore, even when the levels of some of the tumor markers were raised in the serum, the ovarian tumor did not always produce the marker by itself. This study indicates that immunohistochemical identification of a marker in tumor tissue is prerequisite to the use of that marker in the serum to monitor disease status.