Fifteen volunteers underwent intravenous regional anaesthesia on two occasions using 40 ml 0.5% prilocaine, to which had been added either 2 ml 0.9% saline or 0.1 mg fentanyl (resultant concentration 2.5 micrograms/ml). There was no difference in the rate of onset of blockade of cold sensation from an ethyl chloride spray, or to sharp and touch pinprick sensation in either group. There was an increase in the incidence of nausea after tourniquet release in the fentanyl group. It is concluded that the addition of fentanyl 2.5 micrograms/ml to prilocaine 0.5% confers no benefit in intravenous regional anaesthesia.