Pressure-flow study is the gold standard for diagnosis of bladder outlet obstruction (BOO). A prospective study was carried out to compare urodynamic evaluation and measurement of intravesical protrusion of the prostate for diagnosing BOO. Patients presenting with lower urinary tract symptoms (LUTS) associated with benign prostatic hyperplasia and suspected BOO were prospectively evaluated through conventional urodynamics and classified according to the bladder outlet obstruction index (BOOI). They also underwent abdominal ultrasound measurement of the intravesical prostatic protrusion (IPP) and prostatic volume. The IPP was classified into three stages: grade I under 5 mm; grade II, between 5 and 10 mm; and grade III over 10 mm. Forty-two patients, mean age 64.8 +/- 8.5 years were enrolled. Transabdominal ultrasound determined a mean prostatic volume of 45 +/- 3.2 mL. Achieved IPP's values were the following: grade I - 12 (28.5%), grade II - 5 - (12%) and grade III - 25 (59.5%). The results of prostate volume differed significantly between obstructed and non-obstructed men (p = 0.033) and for IPP among obstructed, inconclusive and non-obstructed men (p = 0.016). For IPP, the area under ROC curve was 0.758 (95% confidence interval - 0.601 to 0.876), and the cutoff point to indicate BOO was 5 mm with 95 % sensitivity (75.1 - 99.2) and 50 % specificity (28.2 - 71.8). IPP and prostatic volume measured through abdominal ultrasound are noninvasive and accessible methods that significantly correlate to urinary BOO, and are useful in the diagnosis of male urinary obstructive problems.