PURPOSE: To determine whether the preoperative residual urine fraction(RF) correlates with the change in the maximal flow rate(Qmax) in patients treated for benign prostatic hyperplasia(BPH). MATERIALS AND METHODS: From February 1995 to June 1999, 81 men aged 54 to 82 years (mean: 65.4 +/- 6.7 years) with BPH were examined with Qmax and residual volume using uroflometry and ultrasonography, respectively, before and after transurethral resection of the prostate. The RF was defined as the postvoiding residual/premicturition volume x100. Statistical analysis was performed by simple linear regression using SPSS. RESULTS: The magnitude of improvement in Qmax postoperatively was greatest in patients who had preoperative Qmax < 12mL/sec and RF > 25, followed by Qmax < 12mL/sec and RF < 25, Qmax > 12mL/sec and RF > 25, and Qmax > 12mL/sec and RF < 25. The extent of improvement in Qmax in patients had a value < 12mL/sec was significantly better than that in patients who had a value > 12mL/sec. Among patients who had a Qmax > 12mL/sec preoperatively, the improvement in those who had an RF > 25 was significant better than that in patients who had an RF < 25. CONCLUSION: We suggest that RF and Qmax may have the value as indicator for surgery of benign prostatic hyperplasia preoperatively. The higher RF and Qmax, the poorer postoperative Qmax are anticipates in the patients with benign prostatic hyperplasia.