The study consists of two periods, an initial treatment period and an extended treatment period. Patients who meet all eligibility criteria will receive: Lenalidomide 25 mg p.o. once daily on days 1-21 plus Dexamethasone 40 mg p.o. once daily on days 1, 8, 15, and 22 of each 28-day cycle (4-weeks cycles) until CR achievement, progression of disease or intolerable toxicity. The daily dose of Lenalidomide and that of Dexamethasone will remain fixed. No dose adaptations are allowed according to the hematologic toxicity. However, in case of non-hematologic grade > 2 toxicity, the next cycle may be delayed up to 21 days after the onset of the event until the adverse event has returned to baseline or ≤ grade 1. Patients may be allowed to receive a pre-treatment with high doses of dexamethasone for initial hyperleukocytosis at the discretion of the investigator. The use of hematopoietic growth factor (HGF) during the initial or extended treatment period is also let at the discretion of the investigator. At the end of the initial treatment period, all responding patients (without HLA compatible donor) or patients showing sufficient clinical activity will be allowed to receive extended treatment with cycles identical to the first cycle. Patients may continue to receive treatment as long as the patient is tolerating the treatment well and the patient's physician believes the patient is receiving some benefit. The severity of adverse events will be graded according to the World Health Organization (WHO) criteria. Any life-threatening adverse effects that are observed during the initial or extended treatment period, which are possibly or probably related to study drug, will require the subject to discontinue study treatment.