A method is presented for the measurement of estrone and estradiol in human urine, using electron-capture gas-liquid chromatography. The 3-methyl ether, 17-HFN derivative of estradiol is used for quantification and less than 50 ng/day of estrone and estradiol can be measured by this approach. Such sensitivity enables adequate measurement of these estrogens in urine from post-menopausal women, adult men and preadolescent children. In menstruating women, estrone and estradiol excretion agreed with previously reported values. In adult men, E1 and E2 excretion averaged 2150 and 820 ng/day, respectively, and were twice the levels seen in the urine of post-menopausal women. In preadolescent children, estrone excretion averaged 50 ng/day but estradiol was undetectable (less than 20 ng), except in 2 girls with precocious puberty.