To analyze the performance of basal 17OH-progesterone (17OHP) levels versus the basal 17OHP/cortisol ratio in nonclassical congenital adrenal hyperplasia (NCAH) and polycystic ovary syndrome (PCOS) differential diagnosis. Basal 17OHP levels >10 ng/mL have been used to confirm NCAH diagnosis without the ACTH test; however, best cutoff value is a matter of debate.A cross-sectional study was performed at the endocrinology and gynecological endocrinology outpatient clinics of a tertiary hospital. A total of 361 patients with PCOS (25.0±5.3 years old) and 113 (19.0±13.6 years old) patients with NCAH were enrolled. Basal and ACTH-17OHP levels were measured by radioimmunoassay (RIA), and CYP21A2 molecular analysis was performed to confirm hormonal NCAH diagnosis. Receiver operating characteristic curve analysis compared basal 17OHP levels and the 17OHP/cortisol ratio between NCAH and PCOS patients.Basal 17OHP levels were higher in NCAH patients than in those with PCOS [8.85 (4.20-17.30) vs 1.00 (0.70-1.50) ng/mL, p<0.0001), along with 17OHP/cortisol ratio [0.86 (0.47-1.54) vs 0.12 (0.07-0.19), p<0.0001, respectively]. Basal 17OHP levels and the 17OHP/cortisol ratio were strongly correlated in both groups (rho=0.82, p<0.0001). Areas under the curves for basal 17OHP levels (0.9528) and the 17OHP/cortisol ratio (0.9455) were not different to discriminate NCAH and PCOS (p>0.05). Basal 17OHP level >5.4 ng/mL and 17OHP/cortisol ratio >2.90 had 100% specificity to identify NCAH.Basal 17OHP levels >5.4 ng/mL can be used to perform differential diagnoses between NCAH and PCOS, dismissing the ACTH test. The basal 17OHP/cortisol ratio was not superior to basal 17OHP levels in this scenario.A comparison of the efficiency between basal 17OHP levels and the 17OHP/cortisol ratio to discriminate between patients with NCAH and those with PCOS depicted that both were equally useful for this differentiation.