OBJECTIVE: To examine whether thin endometria can be improved by increasing uterine radial artery (uRA) blood flow. DESIGN: A prospective observational study. SETTING: University hospital and city general hospital. PATIENT(S): Sixty-one patients with a thin endometrium (endometrial thickness [EM] or=0.81). INTERVENTION(S): Vitamin E (600 mg/day, n = 25), l-arginine (6 g/day, n = 9), or sildenafil citrate (100 mg/day, intravaginally, n = 12) was given. MAIN OUTCOME MEASURE(S): EM and RA-RI were assessed by transvaginal color-pulsed Doppler ultrasound. RESULT(S): Vitamin E improved RA-RI in 18 (72%) out of 25 patients and EM in 13 (52%) out of 25 patients. L-arginine improved RA-RI in eight (89%) out of nine patients and EM in six (67%) patients. Sildenafil citrate improved RA-RI and EM in 11 (92%) out of 12 patients. In the control group (n = 10), who received no medication to increase uRA-blood flow, only one (10%) patient improved in RA-RI and EM. The effect of vitamin E was histologically examined in the endometrium (n = 5). Vitamin E improved the glandular epithelial growth, development of blood vessels, and vascular endothelial growth factor protein expression in the endometrium. CONCLUSION(S): Vitamin E, l-arginine, or sildenafil citrate treatment improves RA-RI and EM and may be useful for the patients with a thin endometrium.