Serum levels of CA125 and 90K, a new tumor-associated antigen, were measured in 73 ovarian cancer patients and 70 patients with benign gynecological conditions. Abnormally high serum CA 125 and 90K levels were found in 49% and 78% of these patients, respectively. When a combination of the two markers was used, the sensitivity increased to 86%. The percentage of 90K positivity did not significantly correlate with stage of disease or with histological type. A statistically significant correlation was found between 90K positivity rate and grade of tumor differentiation. Elevated serum CA 125 and 90K levels were present in 32% and 29% of patients with benign ovarian tumors, respectively. Only 1 case out of 26 with endometriosis had high 90K levels, compared to 7 patients with abnormal CA 125 levels. Serial measurements of 90K concentrations were found to be correlated with the clinical behaviour of the disease during chemotherapy. Moreover, rising levels of 90K preceded the clinical detection of recurrence with a median lead-time of 3 months. In 2 of these cases the serial changes in 90K values were better correlated with the course of disease than changes in CA 125 levels. Our data suggest that 90K combined with CA 125 may be applied for the detection and monitoring of ovarian cancer.