The aim of this prospective study was to compare the diagnostic accuracy of detrusor wall thickness (DWT), free uroflowmetry, postvoid residual urine, and prostate volume (index tests) with pressure-flow studies (reference standard) to detect bladder outlet obstruction (BOO) in men. During a 2-yr period, men older than 40 yr with lower urinary tract symptoms and/or prostatic enlargement had the following tests: ultrasound measurements of DWT, free uroflowmetry (Q(max), Q(ave)), postvoid residual urine, and prostate volume. Pressure-flow studies were used to divide obstructed from nonobstructed bladders. One hundred sixty men between 40-89 yr of age (median: 62 yr) were included in the study; 75 patients (46.9%) had BOO according to pressure-flow studies. The results of all investigated index tests differed significantly between obstructed and nonobstructed men. DWT was the most accurate test to determine BOO: the positive predictive value was 94%, specificity 95%, and the area under the curve of ROC analysis 0.93. There was an agreement of 89% between the results of DWT measurement and pressure-flow studies. Measurements of DWT can detect BOO better than free uroflowmetry, postvoid residual urine, or prostate volume. In clinical routine, DWT measurements can be used to judge BOO noninvasively. European Association of Urology.