Recent studies reported that in polycystic ovary syndrome (PCOS) patients, other stimulation agents are superior to the popular first-line regimen, clomiphene citrate (CC) for ovarian stimulation. Nonetheless, CC is still widely used since it is not clear which patients will not respond to it. Furthermore, the prognostic value of endometrium thickness at midcycle is controversial. We aimed to find factors predicting the response to CC and the prognostic value of endometrial thickness at midcycle. We collected data retrospectively from 89 anovulatory PCOS patients who had the first stimulation with 50 mg CC. We analyzed the basal levels of AMH, testosterone, LH, LH:FSH ratio and the endometrial thickness at midcycle by univariate, followed by multivariate regression. The outcome measures were pregnancy, follicle maturation and endometrial thickness at midcycle. Stimulation with 50 mg CC resulted in follicle maturation in 50.6% of the women and in 27.0% pregnancies. In the univariate analysis, greater endometrial thickness, lower LH and AMH levels and a lower LH:FSH ratio were associated with pregnancy (p < 0.05). In the multivariate analysis, only endometrial thickness remained predictive (p = 0.045). The endometrial thickness cutoff level of ≥ 8 mm showed a sensitivity of 87.5% (96% CI 67.6-97.3) and a specificity of 66.7% (95% CI 43.0-85.4) for prediction of pregnancy. In the multivariate analysis AMH levels 5.4 (3.4; 7.0) (ng/mL) predicted pregnancy (β = - 0.194 ± 0.092; p = 0.034) CONCLUSION: We suggest to refrain from CC as first-line regimen in patients with AMH > 7 ng/ml. Under CC treatment, the cutoff value of ≥ 8 mm endometrium thickness at midcycle is associated with a better outcome.