Notwithstanding the potential public health impact of the polycystic ovary syndrome ( PCOS), estimates regarding its prevalence are limited and unclear. Between July 1998 and October 1999, 400 unselected consecutive premenopausal women ( 18 - 45 yr of age) seeking a preemployment physical at the University of Alabama at Birmingham were studied ( 223 Black, 166 White, and 11 of other races). Evaluation included a history and physical examination, a modified Ferriman-Gallwey hirsutism score, and serum screening for hyperandrogenemia, hyperprolactinemia, and 21-hydroxylase-deficient nonclassical adrenal hyperplasia. PCOS was diagnosed by the presence of the following: 1) oligoovulation, 2) hyperandrogenemia and/or hirsutism ( modified Ferriman-Gallwey score greater than or equal to 6), and 3) the exclusion of related disorders. Confirmed PCOS was established in those individuals whose evaluation was complete and indicative of PCOS, and possible PCOS was established when the hormonal evaluation was not complete or was unavailable, but the clinical phenotype was otherwise suggestive of the disorder. The individual probability of PCOS in women with possible PCOS was assigned a weight based on the findings in similar subjects whose evaluation was complete, and the total number of PCOS cases arising from these individuals was calculated (i.e. individual probability of PCOS x total number of subjects in the group). The cumulative prevalence of PCOS in our population was 6.6% (26.5 of 400), including 15 subjects among the 347 women completing their evaluation and a calculated prevalence of 11.5 subjects among the remainder. The prevalence rates of PCOS for Black and White women were 8.0 and 4.8%, respectively, not significantly different. These data from a large representative unselected population support the concept that PCOS is the most common endocrine abnormality of reproductive-aged women in the United States.