The CA 125 II assay on the Elecsys(R) 2010 analyzer was evaluated in an international multicenter trial. Imprecision studies yielded within-run CVs of 0.8-3.3% and between-day CVs of 2.4-10.9%; CVs for total imprecision in the manufacturer's laboratory were 2.4-7.8%. The linear range of the assay extended to at least 4500 kilounits/L (three decades). Interference from triglycerides (10.3 mmol/L), bilirubin (850 micromol/L), hemoglobin (1.1 mmol/L), anticoagulants (plasma), and several widely used drugs was undetectable. Method comparisons with five other CA 125 II assays showed good correlation but differences in standardization. A 95th percentile cutoff value of 35 kilounits/L was calculated from values measured in 593 apparently healthy (pre- and postmenopausal) women. In 95% of patients with benign gynecological diseases CA 125 was </=190 kilounits/L; 63% of patients with newly diagnosed ovarian carcinoma had values >190 kilounits/L. A comparison of CA 125 values obtained with the Elecsys test and with other common CA 125 tests in monitored patients being treated for ovarian cancer showed identical patterns. In conclusion, the Elecsys CA 125 II assay is linear over a broad range, yields precise and accurate results, is free from interferences, and compares well with other assays.