Aims: To test the hypothesis that improvements of lower urinary tract symptoms (IPSS) upon treatment with an ablocker are due to reduction of bladder outlet obstruction (assessed as the bladder outlet obstruction index, BOOI); relationships of either with free flow Q max were also explored. Methods: The database of a large placebo-controlled, randomized, double-blind study with the a-blocker tamsulosin was analyzed retrospectively. Patients were stratified into lower and upper halves according to baseline IPSS, Q max or BOOI and treatment-associated alterations thereof. In these strata differences between values for the other two parameters were analyzed, for example, improvement of IPSS and Q max were compared in patients with below and above median improvement of BOOI. Results: Patients with below and above median baseline for one parameter, for example, IPSS had rather similar values for the other two parameters, for example, Q max and BOOI. Likewise, patients based upon baseline strata for one parameter had rather similar improvements of the other two parameters. Most importantly, patients with below and above median treatment-associated improvements of one parameter, for example, BOOI exhibited only small if any difference for alterations of the other two parameters, for example, IPPS and Q max . Conclusions: We conclude that IPSS, free flow Q max and BOOI are only loosely related at baseline. More importantly, treatment-induced improvements of these parameters are also only loosely related. These data do question the hypothesis that a-blockers largely improve lower urinary tract symptoms by reducing bladder outlet obstruction and suggest that they may also act independent of prostatic smooth muscle tone.