We investigated the usefulness of CA602, a newly developed serum tumor marker, for ovarian cancer. When the cut-off value was set at 60 U/ml, the overall positive rate of this marker in ovarian cancer was 92%, a slightly high rate relative to CA125 measured at the same time (88%). Considering tumor histology, CA602 revealed a high positive rate of 100% in serous adenocarcinoma, whereas the positive rate was 67% in mucinous adenocarcinoma. However, the positive rate was relatively high in benign diseases such as endometrial cysts (64%) and benign ovarian tumors (29%). It is concluded that CA602 is a tumor marker with low specificity and high sensitivity in general. The definite correlation between CA602 and CA125 in ovarian tumors (R = 0.96) suggests that these markers have certain similarities. Thus, CA602 may be a useful serum tumor marker for ovarian cancer as a substitute for CA125.