Analysis of the pressure/flow relation renders objective and detailed information on bladder outlet obstruction. The benefit of pressure/flow analysis for clinical and fundamental research questions, however, cannot be acknowledged without comparison of the different methods that exist. We compared one parameter analysis (URA) with two parameter (PURR) analysis in 99 consecutive patients with benign prostatic enlargement. The normal (instantaneous intrapatient) variability of both the PURR parameter Pvoid,, (minimal pressure during voiding) and the URA is = 10-15 cm H,O. Within these limits agreement between the two methods of analysis in the quantification of (minimal) outlet obstruction was observed in about 50% of the cases. However, when Qmax is less than 6 ml/s (in 49.5% of the patients) the URA number exceeds the value Pvoid,, in 96% of the cases. Predominantly this is caused by the fact that in the majority of these cases the type of bladder outlet obstruction is more constrictive than the URA curve, based on Pdef at Q, , , indicated. In patients with a low flow rate and/or a constrictive type of obstruction, the Pvoid,, resulting from PURR analysis indicates a lower minimal pressure during voiding compared to URA.