We conducted several studies to assess the ability of laboratories to reproducibly measure plasma levels of estrone, estradiol, percent free estradiol, sex hormone binding globulin, dehydroepiandrosterone sulfate, androstenedione, testosterone, progesterone, follicle-stimulating hormone, and prolactin in postmenopausal women. We sent four to seven replicate samples of plasma to each of four well-established endocrine laboratories in the United States on one or two separate occasions. All replicate samples were handled identically during processing, storage, and retrieval, and were labeled to preclude their identification by the receiving laboratory. The within-person coefficient of variation, a measure of laboratory error, was consistently low (< 15%) for follicle-stimulating hormone and prolactin. For estrone and estradiol, hormones present at low levels in postmenopausal women, the laboratory error was often large (> 25%) and the ratio of between-person variation to laboratory error was often less than 2.0. Reproducibility of androstenedione, dehydroepiandrosterone sulfate, testosterone, progesterone, sex hormone binding globulin, and percent free estradiol was also variable. Our results emphasize the need for evaluating laboratory performance before sending samples for hormone analysis and for increased collaboration between epidemiologists and laboratory investigators in improving hormone measurements for use in epidemiological research.