In a prospective study, CA 125 and CA 19.9 serum levels were measured in 229 patients with ovarian cancer [121 with active disease, 108 in complete remission (CR)], and in 20 patients with other malignancies. Abnormal levels of CA 125 were found in 90% of patients with active ovarian cancer, in 1.8% of those in CR and in 38% of cases with other malignancies. Abnormal CA 19.9 serum levels were found in 36, 9 and 48% of these groups, respectively. Serum levels of both tumor markers were related to tumor stage and histological type. The highest levels of CA 125 were found in serous adenocarcinoma and the lowest in the mucinous type (p < 0.0001). In contrast, significantly higher CA 19.9 values were found in mucinous carcinoma than in other histologies (p < 0.0001). CA 125 and CA 19.9 were useful for monitoring disease activity in 88.3 and 32%, respectively, while one or other tumor marker was useful in 92% of patients. At the time of the second-look operation, abnormal CA 125 serum levels were found in 32% (6/19) of patients with active disease and in none of those with CR (0/38). CA 125 sensitivity was 83% (5/6) in those patients with residual tumor > 2 cm and in 8% (1/13) in those with tumor < 2 cm. CA 19.9 values were abnormally high in 16% of cases with persistent disease and in 11% of CR patients. In conclusion, our results confirm that CA 125 is a useful marker in ovarian carcinoma. CA 19.9 improves the results obtained with CA 125 alone only in mucinous adenocarcinomas.