The original CA 125 serum tumor marker test is a homologous double-determinant (OC 125 monoclonal antibody based) assay for the quantification of tumor associated mucin-like CA 125 molecules present in the serum. Commercial kits, now supplied by various manufacturers (and in different versions, e.g. IRMA, EIA, etc.) are currently widely applied in the following clinical situations: (i) Monitoring of disease. Doubling or halving of CA 125 serum values correlated (in 87% of all cases) with tumor progression or regression, respectively. (ii) Early prediction of outcome. Deviation from the ideal CA 125 regression curve predicts poor outcome within 3 months of cytostatic treatment. (iii) Tumor status after completion of therapy. Patients with CA 125 > 35 U/ml have (in 95% of all cases) still tumor present (at second look surgery). However, patients with CA 125 < 35 U/ml have in 50% (mostly minimal) residual disease. (iv) Early detection of recurrence. After a complete remission, a rise in CA 125 precedes tumor recurrence in 75% of all patients, with lead times up to more then 1 year, surpassing the CT-scan in cheapness and accuracy. (v) Diagnosis and differential diagnosis. Only when used in combination with other markers, do CA 125 determinations have a value as a diagnostic adjunct in the discrimination of ovarian cancer patients from those with benign ovarian tumors and from those with advanced colon cancer. Today, optimal management of ovarian cancer patients can only be provided using the CA 125 serum test.