Introduction: In this study the obstructive effect of catheter diameter used in the pressure-flow study in patients with benign prostatic hyperplasia (BPH) who suffer from bladder outlet obstruction had been examined. Materials and Methods: The patients with symptomatic BPH were underwent pressure-flow study by using 8F urethral catheter following free flow measurements. The pressure flow study was repeated by using 4.5F urethral catheter for the patients established bladder outlet obstruction. Twenty-eight patients with a mean age at 66.3 years were chosen for the study group. The measurements of voided volume, flow time, postvoid residual volume (PVR), maximal flow rates (Qmax), detrusor pressure at maximal flow (PdetQmax) and urethral resistance factor (URA) were compared. Results: The results of pressure-flow study by using 8F urethral catheter were significantly different than the results of pressure-flow study by using 4.5F urethral catheter and free flow parameters in means of all parameters except voided volume and postvoid residual volume. Especially, maximal flow rates were significantly lower and flow time was significantly longer on pressure versus free flow studies. No significant difference was found in maximal flow rates between the usage of 4.5F catheters and free flow. By using small sized catheter, 6 (21.4%) of the 28 patients were found as unobstructed according to the Griffith's nomogram (under 30 cmH20 URA). Conclusion: The obstructive effect of the catheter used in pressure-flow study on the patients with bladder outlet obstruction increases proportionately to the diameter of the catheter.