['Serum E2 must be monitored for aromatase inhibitor (AI) therapy, but conventional assays lack sensitivity.', 'Forty amenorrheic breast cancer patients scheduled for AI treatment but requiring hormonological confirmation of their menopausal status were studied. Serum E2 data generated by high-sensitivity RIA and by LC-MS/MS were analyzed for correlation.', 'RIA gave a higher E2 value than LC-MS/MS in 62% of cases, but there was a significant positive correlation. Patients whose E2 levels by RIA were ≥ 2.5 pg/mL higher than those by LC-MS/MS (RIA-H group) and all other patients (RIA-N group) were compared. Both groups showed strong correlations between the two assay methods. With both methods patients with a high BMI had significantly elevated E2. Multiple regression analysis used age, age at menarche, number of births and BMI as explanatory variables. Significant variables were the BMI with LC-MS/MS, and both BMI and age with RIA. The RIA-H and RIA-N groups showed no difference in regard to the BMI, whereas the age was significantly lower in the RIA-H group.'] Is assay of serum E2 concentration in postmenopausal breast cancer patients using a high-sensitivity RIA method generally useful?