Inclusion Criteria:

• Understand and voluntarily sign an informed consent form.
• Age >=18 years at the time of signing the informed consent form.
• Able to adhere to the study visit schedule and other protocol requirements.
• AML (>/= 20% blasts) including secondary (s)AML (after radio-chemotherapy) with karyotype abnormalities involving monosomy 5 or del5q or MDS and sMDS RAEB-1 and RAEB-2 with karyotype abnormalities involving monosomy 5 or del5q or MDS and sMDS type RA(+/-RS) or RCMD(+/-RS) only with complex karyotype abnormalities involving monosomy 5 or del5q
• in complete hematological remission documented by bone marrow aspiration within 8-12 weeks after allogeneic HSCT
• All previous cancer therapy, including radiation, hormonal therapy and surgery, must have been discontinued at least 4 weeks prior to treatment in this study.
• ECOG performance status of </= 2 at study entry.

• Laboratory test results within these ranges:

  • Absolute neutrophil count >= 1.0 x 10 9/L
  • Platelet count >= 100 x 10 9/L
  • Serum creatinine <= 2.0 mg/dL
  • Total bilirubin <= 1.5 mg/dL
  • AST (SGOT) and ALT (SGPT) <= 5 x ULN
• Females of childbearing potential (FCBP)† must agree to use two reliable forms of contraception simultaneously or to practice complete abstinence from heterosexual intercourse during the following time periods related to this study: 1) for at least 28 days before starting study drug; 2) while participating in the study; and 3) for at least 28 days after discontinuation from the study. The two methods of reliable contraception must include one highly effective method (i.e. intrauterine device (IUD), hormonal [birth control pills, injections, or implants], tubal ligation, partner's vasectomy) and one additional effective (barrier) method (i.e. latex condom, diaphragm, cervical cap). FCBP must be referred to a qualified provider of contraceptive methods if needed.
• Disease free of prior malignancies for >= 5 years with exception of currently treated basal cell, squamous cell carcinoma of the skin, or carcinoma "in situ" of the cervix or breast
• Able to take aspirin (ASA) 100mg daily as prophylactic anticoagulation in case of concomitant steroid treatment (patients intolerant to ASA may use low molecular weight heparin).

Exclusion Criteria:

• Any serious medical condition, laboratory abnormality, or psychiatric illness that would prevent the subject from signing the informed consent form.
• active uncontrolled acute GVHD overall grade 3-4
• Pregnant or breast feeding females. (Lactating females must agree not to breast feed while taking lenalidomide).
• History of arterial or venous embolism or stroke
• Any condition, including the presence of laboratory abnormalities, which places the subject at unacceptable risk if he/she were to participate in the study or confounds the ability to interpret data from the study.
• Use of any other experimental drug or therapy to treat MDS or AML within 28 days of baseline (patients within a clinical trial evaluating new conditioning regimens are allowed to participate in the LENAMAINT study)
• Known hypersensitivity to thalidomide or lenalidomide.
• history of erythema nodosum if characterized by a desquamating rash while taking thalidomide or similar drugs.
• Known positive for HIV or infectious hepatitis, type A, B or C.