A Japanese woman had a mucinous cystadenoma of the left ovary associated with very high serum levels of CA19-9 (3170 u/ml) and CA125 (1705 u/ml). Such high levels of both antigens have not been reported previously in patients with benign ovarian mucinous cystadenoma. Immunostaining demonstrated CA19-9 and CA125 in the tumor, and different patterns of expression were noted depending on the differentiation of the epithelium. The postoperative tumor marker profile suggested that no residual tumor was left after the operation.