The progestin-induced withdrawal bleeding test has often been used in patients with amenorrhea to assess endogenous estrogen (E) production. The endometrial thickness measured by vaginal ultrasonography with a 5 MHz transducer is also dependent on E stimulation of the endometrium. In this study, 70 consecutive patients were evaluated with both the progestin-induced withdrawal bleeding test and a measurement of the endometrial thickness by vaginosonography. An endometrial thickness of 1.5 mm or less was selected to predict absence of bleeding after a progesterone (P) challenge test. This resulted in a sensitivity of 94% with a 95% confidence interval (CI) of 0.70 to 1.00 and a specificity of 93% with a 95% CI of 0.82 and 0.98. The positive and negative predictive values were 79% and 98%, respectively. This study shows that the endometrial thickness measured by vaginosonography can predict the results of the P challenge test in patients with amenorrhea.