A device and a method for generating therapeutic foam from a sclerosing solution such as polidocanol and a gas which is absorbable or dissolvable in the body comprises a disposable cassette including a liquid container and a flexible chamber containing gas, chamber being formed as a loop (6). Located in the loop chamber (6) is a foam generating mesh (8). In use, liquid is introduced into the chamber and then the cassette is inserted into a pumping machine where it is engaged by a peristaltic pump which circulates the contents of the loop chamber (6) through the mesh (8), thus creating foam. When a steady state is reached, foam may be withdrawn using a syringe via either a port (3) or septum.