Intravenous cannulation is a necessary procedure in many hospital patients. Some patients are regarded as having 'impossible' veins that are invisible and impalpable despite venous tourniquet. We investigated the use of a hand-held Doppler to identify veins suitable for percutaneous cannulation in such patients. A total of 24 arms in 12 patient volunteers with invisible and impalpable forearm veins was studied by squeezing the ipsilateral hand and using a hand-held 8 MHz Doppler probe on the forearm. Each venous signal was noted and the loudest chosen. Classification as 'good' or 'poor' was made on signal strength. The best signal was then mapped and the direction was indicated by a line on the skin. Each forearm was examined using a colour-flow Duplex. All veins were noted and diameters measured with and without tourniquet. These results were compared with the best veins found by hand-held Doppler. In all, 23 (96%) arms had the largest vein correctly identified by the hand-held Doppler. All 19 (100%) described as 'good' on signal had a diameter of 2.0-4.2 mm (mean 3.2 mm). All 4 (100%) described as 'poor' on Doppler signal had diameters of 1.4-1.7 mm (mean 1.6 mm). Hand-held Doppler, used as described, can accurately identify forearm veins larger than 2.0 mm in diameter in patients with invisible and impalpable veins with venous tourinquet. These veins should be amenable to percutaneous cannulation.