In patients with symptoms suggestive of bladder outflow obstruction and a free flow rate less than 12 ml/s a visual analogue scale was used to measure the response to the question "Compared to 5 years ago, how thick is your stream when you pass urine?". The visual analogue scale compared favourably with cystometry in distinguishing between low flow rates resulting from impaired detrusor contractility and low flow rates due to bladder outflow obstruction. A visual analogue scale may serve to alert the surgeon to the presence of impaired detrusor contractility and the need for a fuller urodynamic assessment in patients with a low urinary flow rate prior to consideration for prostatectomy.