OBJECTIVE We sought to assess the accuracy of endometrial thickness measurement in the diagnosis of endometrial cancer in patients with obesity, diabetes, and hypertension and to evaluate whether patient characteristics influence endometrial thickness irrespective of the final diagnosis. METHODS This was a prospective study of women not using hormone replacement therapy who presented with postmenopausal bleeding at 8 hospitals in The Netherlands. All women underwent transvaginal ultrasonography and, in the event that the endometrial thickness (double layer) was more than 4 mm, subsequent endometrial sampling. The performance of endometrial thickness measurement in the diagnosis of atypical hyperplasia and endometrial cancer was evaluated in subgroups of patients with diabetes, hypertension, and obesity by using receiver operating characteristic analysis. RESULTS Overall, we included 594 consecutive women, of whom 62 (10%) had endometrial carcinoma and 6 (1%) had atypical hyperplasia. In these women, transvaginal ultrasonography had an area under the receiver operating characteristic curve of 0.87 (standard error [SE] 0.03). In the absence of (pre)malignancy, women with diabetes or obesity were found to have thicker endometria than women without these risk factors, whereas in women with a (pre)malignancy, this difference was not present. The area under the receiver operating characteristic curve decreased to 0.74 (SE 0.05) and 0.75 (SE 0.07) in diabetic women and obese women, respectively. The presence or absence of hypertension had no impact on the accuracy of transvaginal ultrasonography. CONCLUSION In view of the decreased diagnostic accuracy in diabetic women and obese women, the clinical value of transvaginal endometrial thickness measurement in these women is questionable.