The analytical and clinical performance of a commercial automated immunoassay system (Immulite) for estradiol (E2) in serum was evaluated. The functional sensitivity for E2 was 0.07 nmol/l, and analytical imprecision (<13%, <9% and <7% at 0.22, 0.51 and 1.51 nmol/l, respectively) for concentrations above this detection limit met published analytical goals. The assay recovery was good and the assay was linear over a wide concentration range. No sample carryover was found, and interferences from common substances present in serum were observed only at very high concentrations. Most of samples from men and postmenopausal women showed E2 concentrations below the detection limit. Longitudinal estradiol profiles from 11 healthy menstruating women showed characteristic menstrual cycle patterns (12 samples per subject obtained during a 30-day period). Longitudinal studies on women during induction of ovulation showed that E2 concentrations are highly correlated with the total number of follicles. Our results demonstrate the reliability of this system for routine use in the clinical laboratory.