CA 125 is currently widely applied in the management of patients with ovarian cancer. However, a change in results of CA 125, which should be considered significant, has not been defined. The aim of this study was to investigate the ability of CA 125 to signal progressive ovarian cancer during follow-up after firstline chemotherapy. The study patients were selected retrospectively among 255 patients with stage IC-IV ovarian cancer. The evaluation of the CA 125 information was based on the analytical imprecision, the normal intra-individual biological variation, the sampling interval, and the cut-off value. Additionally, the utility of a new assessment criterion based upon an increment of 2.5 times the baseline CA 125 concentration confirmed by a third measurement was investigated. The efficiency of CA 125 to identify progression and non-progression during follow-up varied between 76.5 and 79.9%, depending on the applied time limit for an acceptable positive lead time. The median lead time for true positive results was 95-99.5 days. Using the new elaborated criterion, the efficiency of CA 125 for identifying progression and non-progression varied between 75.7 and 78.5%, depending on the applied time limit for an acceptable positive lead time. The median lead time for true positive results was 91-95.5 days. CA 125 provided early and reliable information about progressive disease during follow-up. The applied criteria can therefore be recommended in further studies assessing the clinical utility of serological tumor markers in patients with ovarian cancer.