Polycystic ovary syndrome (PCOS) affects about 4 to 6% of women of reproductive age and accounts for at least 75% of hyperandrogenic patients. PCOS is diagnosed by the presence of oligo-ovulation and hyperandrogenism after the exclusion of related disorders, such as 21-hydroxylase-deficient nonclassic adrenal hyperplasia (NCAH). In turn, NCAH is a homozygous recessive disorder, diagnosed by a corticotropin-stimulated 17-hydroxyprogesterone (17-HP) level greater than10 ng/mL (30.3 nmol/L) and confirmed by genotyping of the CYP21 gene. The prevalence of NCAH is approximately 50 times less than that of PCOS, affecting between 1 and 10% of hyperandrogenic women, depending on ethnicity. However, it is generally difficult to distinguish NCAH from PCOS solely on clinical grounds, as both demonstrate varying degrees of hyperandrogenism and ovulatory dysfunction. Most PCOS patients have insulin resistance, in contrast to those with NCAH. Likewise, polycystic ovaries are observed in up to 40% of NCAH patients. Both disorders have a strong familial component. The only method that allows the separation of NCAH from PCOS patients is the measurement of 17-HP levels. In conclusion, PCOS and NCAH have differences in prevalence and pathophysiology. However, because the disorders have significant clinical and hormonal similarities, the measurement of 17-HP, preferably basally as a screening method, should be incorporated into the evaluation of all hyperandrogenic patients.