Objective: To test the hypothesis that women with polycystic ovary syndrome (PCOS) are distinguishable from those with 21-hydroxylase-deficient nonclassic adrenal hyperplasia on the basis of having polycystic ovaries and metabolic dysfunction. Design: Prospective observational. Setting: Tertiary care center. Patient(s): Fifty-two lean and 54 obese women with PCOS according to the 1990 National Institutes of Health criteria, 23 women with nonclassic adrenal hyperplasia, and 27 controls. Intervention(s): History and physical examination, blood sampling, ovarian sonography, oral glucose tolerance, and acute adrenocorticotropin stimulation testing. Main Outcome Measure(s): The frequency of clinical, biochemical, and metabolic features. Result(s): Women with PCOS had a higher frequency of oligomenorrhea or amenorrhea than those with nonclassic adrenal hyperplasia. Mean androstenedione and DHEAS levels were highest in nonclassic adrenal hyperplasia. The degree of metabolic dysfunction was greatest in obese women with PCOS; women with nonclassic adrenal hyperplasia and lean women with PCOS did not differ in degree of metabolic dysfunction. Women with nonclassic adrenal hyperplasia had a lower prevalence of polycystic ovaries than those with PCOS. The proportion of patients with an LH/FSH ratio >2 was greater in women with PCOS, compared with those with nonclassic adrenal hyperplasia. Basal 17-hydroxyprogesterone levels >2 ng/mL were found in 87%, 25%, 20%, and 7% of women with nonclassic adrenal hyperplasia, lean women with PCOS, obese women with PCOS, and controls, respectively. Conclusion(s): Nonclassic adrenal hyperplasia should be excluded in all women presenting with hirsutism, with use of a basal follicular phase 17-hydroxyprogesterone level, regardless of the presence of polycystic ovaries or metabolic dysfunction; however, women with nonclassic adrenal hyperplasia have a higher prevalence of normal ovulation and lower likelihood of having an LH/FSH ratio >2 or polycystic ovaries. © 2010 American Society for Reproductive Medicine.