Prostatic urethral obstruction as defined by urodynamic criteria is only present in about 70% of patients undergoing surgical treatment for benign prostatic hyperplasia (BPH). Infravesical obstruction can only be diagnosed by simultaneous recording of detrusor pressure (intravesical pressure minus abdominal pressure) and urinary flow rate. The interpretation of these recordings should be based upon the pressure-flow relationship. The possible clinical implications of the objective diagnosis of prostatic urethral obstruction in BPH are discussed.