Objective To study the relationship between the degree of bladder outlet obstruction (BOO), detrusor contractility and residual urine in patients suffering from benign prostatic hyperplasia (BPH).Methods In 181 patients with BPH, degree of BOO, detrusor contractility, residual urine caculated from cathetering combined with the difference between the filling and the voiding were recorded and analysized statistically using urodynamic technique.Results Residual urine increased when the detusor contractility was weakened (F=12.134, P=0.001). In patients wih severe BOO, there was no significant difference in residual urine (F=2.386, P=0.071).Conclusions Increased residual urine is mainly resulted from decreased detrusor contractility. BOO has no significant influence on residual urine. Some patients with normal or weakened detrusor contractility may have more residual urine