AIMS There is no generally accepted consensus how to evaluate patients with lower urinary tract symptoms (LUTS) suggestive of bladder outlet obstruction (BOO). We have tried to determine whether the most frequently used objective variables as prostate volume, IPS-score, maximum flow rate, residual urine volume, functional bladder capacity, and pressure-flow study are reliable for diagnosis of BOO and we investigated the influence of idiopathic detrusor overactivity (IDO) on this condition. METHODS A total of 153 men with LUTS and suspected BOO were systematically examined with routine investigation including digital rectal examination, transrectal ultrasound (TRUS), post-void residual urine volume measurement, uroflowmetry, and pressure-flow study. All patients completed IPS-score. Patients were divided into groups based on Schäfer's grade of obstruction and incidence of IDO and clinical and urodynamical variables were compared. RESULTS At baseline, 45.8% of the patients were urodynamically moderately obstructed and 37.9% were found to be severely obstructed. The grade of obstruction did not correlate with age. Prostate volume, post-void residual volume (PVR), and maximum flow rate correlated significantly with the degree of obstruction. The mean IPS-score remained almost unchanged throughout all obstruction groups. The incidence of IDO was 40.5% and increased from 16% in the minor obstruction group to 38.6% and 53.4% in the moderate and severe obstruction group, respectively. The patients with IDO were older, had larger prostates and were more obstructed. There was no impact of IDO on symptomatology of BOO. CONCLUSIONS These data indicate that IPS-score does not achieve sufficient diagnostic accuracy and its role in the assessment of BOO is limited. The grade of obstruction is more related to prostate volume, PVR, and maximum flow rate. BOO and IDO seem to be related and have numerous mutual interactions.