Measurement of urinary flow rate was carried out on 39 male patients with bladder outlet obstruction due to benign prostatic hypertrophy. All patients were over 50 years of age. Ten subjects, by clinical evaluation, had doubtful bladder outlet obstruction, nine had mild obstruction and 20 patients had moderate or severe obstruction. Four criteria were used for clinical diagnosis: obstructive symptoms, size of prostate by rectal digital exploration, volume of postmicturition residual urine and degree of bladder trabeculation seen on cystourethroscopy. Considerable inaccuracy results from basing the degree of impairment of urinary flow on the patient's obstructive symptoms, degree of prostatic enlargement, postmicturition residual urine or degree of bladder trabeculation. Severe obstruction symptoms generally indicate impaired urinary flow but the absence of obstructive symptoms does not exclude impaired flow.