Facing an aging society, health care providers are seeing an increase in the number of patients with benign prostatic hyperplasia (BPH), a condition that adversely affects the quality of life (QOL). Since bladder outlet obstruction (BOO), one of the main symptoms of BPH, can lead to acute urinary retention, renal dysfunction, and urinary tract infection, and thereby influence life prognosis, BPH requires proper diagnosis and aggressive treatment. Transrectal ultrasonography (TRUS) has proven extremely useful in evaluating prostatic morphology for diagnosis of BPH. The estimated prostatic volume, presumed circle area ratio, and intravesical prostatic protrusion based on TRUS are useful parameters for predicting the severity of BOO, and recent evidence has further suggested that the resistive index (RI) in the capsular branches of the prostatic artery obtained using Doppler ultrasound correlates with the severity of BOO. While pressure-flow studies (PFS) are now generally considered the most accurate method for diagnosing BOO, their use is limited due to the invasiveness of the procedure. In the present study, RI was relatively low at 0.66 in patients diagnosed as not having BOO based on a PFS, while that in patients with BOO was relatively high at 0.75. Receiver operating characteristic analysis showed that RI was relatively high at 0.80 for the presence/absence of BOO. Consequently, in determining the severity of BOO resulting from BPH, diagnostic accuracy is expected to be further improved by combining TRUS with RI measurements.