In order to reliably diagnose bladder outflow obstruction (BOO) in benign prostatic hyperplasia (BPH), we conducted a comparative study in 76 patients with clinically diagnosed BPH, 52 of whom had BOO and 24 who did not. Urodynamic assessment revealed that urethral opening pressure, minimal urethral opening pressure, detrusor pressure at maximal urine flow, and detrusor pressure were significantly higher in patients with BOO than in patients who did not have BOO or whose assessments1 were normal (P < 0.01); no significant difference between patients without BOO and those who were normal were found. Among the 27 patients who underwent prostatectomy, maximal flow rate rate was significantly higher in 23 patients with BOO than in 4 patients without BOO after surgery (P < 0.01). Patients with and without BOO had similar I-PSS (International Prostate Symptom Score) and PS (prostatic size) These results suggest that I-PSS, PS, free Qm (maximal flow rate) and RUV (residual urine volume) are not specific markers for BOO diagnosis in BPH patients. Besides, bladder neck pressure, bladder neck length, prostatic urethral pressure, and prostatic urethral length of static urethral pressure profile (SUPP) were significantly higher in BPH patients, compared to normal. However, there was no significant difference in SUPP between patients with and without BOO. We concluded that P/F (prostate and urine flow) study and SUPP together could provide better guide therapy options and prognosis of BPH.