This study investigates the dose response relationship for the efficacy and safety of BAF312 compared to placebo in active DM patients over a treatment period of 6+6 months and to determine the minimum dose required for a maximal clinical effect. The study is composed of 2 periods: a double-blind period I with BAF312 administered at different daily doses (0.5, 2, 10 mg and placebo) and a fixed-dose Period II in which BAF312 will be administered at the dose of 2 mg daily .