Received: September 8, 2008; Received in revised form: December 15, 2008; Accepted: December 16, 2008; Published Online: February 06, 2009 Endometrial growth and uterine blood flow: a pilot study for improving endometrial thickness in the patients with a thin endometrium Akihisa Takasaki, M.D., Ph.D., Hiroshi Tamura, M.D., Ph.D., Ichiro Miwa, M.D., Toshiaki Taketani, M.D., Ph.D., Katsunori Shimamura, M.D., Ph.D., Norihiro Sugino, M.D., Ph.D. 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] <8 mm) and high radial artery-resistance index of uRA (RA-RI ≥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.