We prospectively estimated the prevalence of the polycystic ovary syndrome (PCOS), as defined by the NIH/NICHHD 1990 endocrine criteria, in a population of 154 Caucasian women of reproductive age reporting spontaneously for blood donation. Anthropometric data; the presence of hirsutism, acne, and androgenic alopecia; and the menstrual history were recorded by a single investigator. In 145 women, blood samples were also obtained for measurement of serum androgen levels. PCOS was defined by the presence of 1) oligomenorrhea, 2) clinical and/or biochemical hyperandrogenism, and 3) exclusion of hyperprolactinemia, thyroid disorders, and nonclassic 21-hydroxylase deficiency. Hirsutism was defined by a modified Ferriman-Gallwey score of 8 or more, acne was considered as a sign of hyperandrogenism when persistent after the second decade of life, and hyperandrogenemia was defined by an increase in circulating testosterone or dehydroepiandrosterone sulfate or an increase in the free androgen index above the 95th percentile of the control values derived from the nonhirsute, nonacneic women having regular menses who were not receiving hormonal therapy. PCOS was present in 10(6.5%), hirsutism was present in 11 (7.1%), and acne was present in 19 (12.3%) of the 154 women. Our results demonstrate a 6.5% prevalence of PCOS, as defined, in a minimally biased population of Caucasian women from Spain. The polycystic ovary syndrome, hirsutism, and acne are common endocrine disorders in women.