A foam for the treatment of varicose veins is disclosed, comprising a sclersosant solution, e.g. polidocanol, foamed with a gas mixture which includes greater than 41 % of a lipid soluble gas such as xenon. The aim is to provide a foam which may be injected into a varicose vein in large quantities because the gas component is dissipated quickly after the foam has acted on the varicose vein.