Prospective multi-center observational (non-interventional) cohort study.Patients will be managed routinely according to the local protocol at each center for the treatment of multiple myeloma. Study period: From initiation of lenalidomide therapy for a period of 18 months. Setting and location: Institute of hematology, Sheba medical center and Hematology Institute at the Davidoff cancer center, Rabin medical center, Israel.