The predictive value of CA 125 assay for recurrence in ovarian cancer patients in follow-up was analyzed in a study from April 1984 through June 1987. Forty-two patients with no evidence of disease (NED), with positive antigen levels at diagnosis and negative at the end of active treatment, were considered eligible for the analysis. Median follow-up time was 16 months (range, 5-34). Outcome analysis revealed 19 cases still NED: 16 had normal CA 125 levels (less than 35 U/ml). The 3 patients with positive antigen titers were intensively investigated with no evidence of recurrence. Twenty-three cases had disease recurrence: 13 of them had elevated marker levels prior to relapse diagnosis, with a median lead time of 5 months (range, 2-13). In contrast, 10 patients had positive titers at or soon after the recurrence. Test sensitivity was therefore 56% and specificity 84%. Predictive value for recurrence of elevated CA 125 levels was 0.81.