This study aimed to determine the relationship between resistive indices (RIs) and changes in prostate size after medical treatment in patients with benign prostatic hyperplasia (BPH).A total of 86 patients with BPH were included in the study, excluding 42 patients with a total prostate volume (TPV) of <30 cc or taking α1-adrenergic blockers and 5α-reductase inhibitors (5ARI) before study participation. Therefore, the data for 44 patients were analyzed. All patients were treated with α1-adrenergic blockers and 5ARIs. The variables examined were prostate-specific antigen, International Prostate Symptom Score, quality of life score, maximal urinary flow rate, residual urine volume, TPV, transition zone volume, and RIs of the urethral artery and left and right capsular arteries. These variables were assessed at baseline and after 3 and 6 months of treatment.The mean TPV was 43.5 ± 10.9 and decreased to 35.2 ± 11.5 and 33.9 ± 9.8 after 3 and 6 months of treatment, respectively (p < 0.001). The mean RI of the urethral artery, right capsular artery, and left capsular artery at pretreatment did not decrease significantly. However, comparing the baseline with 3-month data, TPV at 3 months/TPV at baseline was significantly correlated with RI changes in the left capsular artery (r = 758; P < 0.001).In patients with BPH, α1-adrenergic blocker and 5ARI medications for 3 and 6 months did not result in a significant reduction in the RI of the urethral artery and both capsular arteries. Larger scale, prospective studies are needed to evaluate the relationship between TPV and RI reductions.