We developed a noninvasive method to diagnose bladder outlet obstruction. An ultrasound based decorrelation method was applied in male patients with lower urinary tract symptoms. In 60 patients ultrasound data were acquired transperineally while they were voiding while sitting. Each patient also underwent a standard invasive pressure flow study. High frequent sequential ultrasound images were successfully recorded during voiding in 45 patients. The decorrelation (decrease in correlation) between subsequent ultrasound images was higher in patients with bladder outlet obstruction than in unobstructed patients and healthy volunteers. ROC analysis resulted in an AUC of 0.96, 95% specificity and 88% sensitivity. A linear relationship was fitted to the decorrelation values as a function of the degree of obstruction represented by the bladder outlet obstruction index, measured in the separate pressure flow studies. It is possible to noninvasively diagnose bladder outlet obstruction using the ultrasound decorrelation technique.