To investigate the relationship between body mass index (BMI) and transvaginal ultrasonographic endometrial thickness. Two hundred and twelve postmenopausal Japanese women (mean age, 62.1+/-8.0 years; range 47-85) with histologically proven normal endometrium and with endometrial thickness more than 1.0 mm were studied. Baseline characteristics including age, years since menopause, and BMI were recorded for each subject. The relationship between sonographic endometrial thickness and baseline characteristics was assessed in each subject. BMI was significantly correlated with endometrial thickness (r=0.40, p<0.001), but age and years since menopause were not correlated. On stepwise regression analysis only BMI was still associated with endometrial thickness (R2=0.16, p<0.001). Sonographic endometrial thickness differs with BMI in postmenopausal women. Higher BMI is associated with greater endometrial thickness.