Lack of standardization of high-quality steroid hormone assays is a major deficiency in epidemiologic studies. In postmenopausal women, reported levels of serum 17beta-estradiol (E(2)) are highly variable and median normal values differ by approximately a 6-fold factor. A particular problem is the use of E(2) assays for prediction of breast cancer risk and osteoporotic fractures, where assay sensitivity may be the most important factor. Identification of women in the lowest categories of E(2) levels will likely provide prognostic information that would not be available in a large group of women in whom E(2) levels are undetectable by less sensitive assays. Detailed and costly methods involving extraction and chromatography in conjunction with RIA provide generally acceptable E(2) results in postmenopausal serum, whereas less tedious, direct immunoassays suffer from inadequate specificity and sensitivity. Studies comparing the two types of methods generally report higher E(2) values with the direct methods as a result of cross-reactivity with other steroids and reduced correlation with biological variables such as body mass index. Similar problems exist with measurements of E(2) and estrone in men, and estrone and testosterone in women. Interest in mass spectrometry-based assays is increasing as potential gold standard methods with enhanced sensitivity and specificity; however, these assays require costly instrumentation and highly trained personnel. Taking all of these issues into consideration, we propose establishment of standard pools of premenopausal, postmenopausal, and male serum, and utilization of these for cross-comparison of various methods on an international basis. An oversight group could then establish standards based on these comparisons and set agreed upon confidence limits of various hormones in the pools. These criteria would allow validation of sensitivity, specificity, precision, and accuracy of current steroid hormone assay methodology and provide surrogates until a true gold standard can be developed.