Inclusion Criteria:

1. Patients must be age ≥ 18.

2. Patients must have an ECOG PS ≤ 2.

3. Patients must understand and voluntarily sign informed consent.

4. Able to adhere to the study visit schedule and other protocol requirements.

5. Patients must carry the diagnosis of B-CLL/SLL as per WHO diagnostic criteria.

6. SLL/CLL must be defined as relapsed or refractory disease as defined by the
NCI-sponsored IWG criteria.

- Relapsed CLL/SLL: Relapse is defined as a patient who has previously achieved
CR or PR, but after a period of 6 or more months, demonstrates evidence of
disease progression.

- Refractory CLL/SLL: Refractory disease is defined as treatment failure or
disease progression within 6 months to the last antileukemic therapy.

7. Patients must have received and failed at least one purine-based treatment regimen
(ie. FCR, FR, PCR, Fludarabine) or alemtuzumab-based regimen or bendamustine-based
regimen prior to study enrollment.

8. Serum bilirubin levels <1.5 times the upper limit of the normal range for the
laboratory (ULN). Higher levels are acceptable if these can be attributed to active
hemolysis, ineffective erythropoiesis or Gilbert's disease.

9. Serum glutamic-oxaloacetic transaminase (SGOT) (aspartate aminotransferase [AST]) or
serum glutamic-pyruvic transaminase (SGPT) (alanine aminotransferase [ALT]) levels <2
x ULN. (or <5 x ULN if hepatic metastases are present)

10. Subjects must have calculated creatinine clearance ≥ 30ml/min by Cockcroft-Gault
formula. See section below, "Dosing Regimen", regarding lenalidomide dose adjustment
for calculated creatinine clearance > 30ml/min and < 60ml/min

11. Absolute neutrophil count > 1.0 x 109 / L

12. Platelet count > 50x 109 / L (unless bone marrow is heavily infiltrated with
underlying disease (50% or more)

13. Disease free of prior malignancies for > 5 years with exception of currently treated
basal cell, squamous cell carcinoma of the skin, or carcinoma "insitu" of the cervix
or breast.

14. All study participants must be registered into the mandatory RevAssist® program, and
be willing and able to comply with the requirements of RevAssist®.

15. Able to take aspirin (81 or 325 mg) daily as prophylactic anticoagulation (patients
intolerant to ASA may use warfarin or low molecular weight heparin).

16. Females of childbearing potential (FCBP)† must have a negative serum or urine
pregnancy test with a sensitivity of at least 50 mIU/mL within 10 - 14 days prior to
and again within 24 hours prior to prescribing lenalidomide (prescriptions must be
filled within 7 days) and must either commit to continued abstinence from
heterosexual intercourse or begin TWO acceptable methods of birth control, one highly
effective method and one additional effective method AT THE SAME TIME, at least 28
days before she starts taking lenalidomide. FCBP must also agree to ongoing
pregnancy testing. Men must agree to use a latex condom during sexual contact with a
FCBP even if they have had a successful vasectomy. See Appendix: Risks of Fetal
Exposure, Pregnancy Testing Guidelines and Acceptable Birth Control Methods.

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Exclusion Criteria:

Any serious medical condition, laboratory abnormality, or psychiatric illness that would
prevent the subject from signing the informed consent form.

2. 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.

3. Patients must be lenalidomide / azacitidine naïve prior to study enrollment. 4.
Patient's must not carry a diagnosis of HIV, Hepatitis B or Hepatitis C. Patients who are
seropositive because of hepatitis B virus vaccine are eligible.

5. Patients must not have received chemotherapy, immunotherapy or any experimental study
drug for CLL or SLL at least 4 weeks prior to study enrollment and initiation of
treatment.

6. Patients with history of B-CLL and the development of prolymphocytic leukemia or
Richter's transformation.

7. Known or suspected hypersensitivity to azacitidine, mannitol, thalidomide or
lenalidomide.

8. Pregnant or breast feeding females. Lactating females must agree not to breast feed
while taking lenalidomide.

9. Evidence of laboratory TLS by Cairo-Bishop Definition of Tumor Lysis Syndrome.
Subjects may be enrolled upon correction of electrolyte abnormalities.

10. Patients with advanced malignant hepatic tumors. 11. Concurrent use of other
anti-cancer agents or treatments. 12. Uncontrolled systemic fungal, bacterial, or viral
infection (defined as ongoing signs/symptoms related to the infection without improvement
despite appropriate antibiotics or other treatment);

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