A method of treating restless leg syndrome (RLS) in a patient, said method comprising the steps of withdrawing and bypassing the blood of the patient in a continuous flow into contact with one face of a membrane, simultaneously passing dialysate solution in a continuous flow on the opposite face of the membrane, and returning the blood to the patient,
- wherein the membrane has a MWRO of between about 9.0 kDa and about 12.0 kDa and a MWCO of between about 55 kDa and about 110 kDa as determined by dextran sieving before blood contact of the membrane, and
- wherein the membrane comprises at least one polymer selected from the group of polymers consisting of polysulfone, polyethersulfone and polyarylethersulfone, and further comprising polyvinylpyrrolidone,
- wherein treatment with the membrane reduces one or more RLS symptoms in the patient.