The present invention relates to pharmaceutical composition comprising an iron carbohydrate complex for use in a method for treatment or prevention of Restless Leg Syndrome (RLS) of a human patient, wherein the human patient prior to treatment has a magnetic resonance phase imaging of 0.02 radians above the average value of a control group in the substantia nigra, thalamus, putamen, orpallidum. The invention provides a higher probability for a RLS patient being treated to experience a relief in symptoms.