A flexible sleeve 2 to be worn around an arm and/or a hand has a gas inlet and a gas outlet, with the gas flowing over a wearer's skin. The gas is warmed to promote vasodilation in order that a vein can be more easily located for venipuncture. Preferably apertures at each end of the sleeve can be closed by drawing inwards the sleeve edges. There may be an access portion 58 which can be opened and secured by a magnetic or hook-and-loop fastening； it may be transparent. The sleeve may have an inner gas-permeable wall 52a and an outer impermeable wall 52b, defining between them a chamber 62 through which the gas flows. The gas inlet and outlet may be ports 4 and 6 with a barrier 66 or baffle between them. The gas may be circulated by a portable blower, in a closed or open circuit.
