Lenalidomide and azacitidine each have activity in myelodysplastic syndromes (MDS) patients, where both microenvironment and cell regulatory mechanisms contribute to disease pathogenesis. The objective of this multicenter, Phase 2 expansion trial was to determine the efficacy and safety of combination therapy with azacitidine (75mg/m(2)/d x 5 days) and lenalidomide (10mg/d x 21 days (28-day cycle)) in patients with higher-risk MDS. Among 36 patients enrolled (18 Phase 1, 18 Phase 2), median age was 68 years (range 47-78) and follow-up was 12 months (range 3-55). IPSS categories included Intermediate-1 (n=5 patients with excess blasts), Intermediate-2 (20), and High (11). Common grade 3/4 non-hematologic adverse events included febrile neutropenia (22% of patients), other infection (11%), pulmonary (11%), cardiac (11%), constitutional (11%), and dermatologic (11%). The overall response rate (per modified MDS International Working Group criteria) was 72%: 16 patients (44%) achieved a complete response (CR), 10 (28%) had hematologic improvement. Median CR duration was 17+ months (range, 3-39+); median overall survival was 37+ months (range, 7-55+) for CR patients, 13.6 months for the entire cohort (range, 3-55). TET2/ DNMT3A/ IDH1/2 mutational status was associated with response in a limited number of patients. The lenalidomide/azacitidine combination is well-tolerated and highly active in treating higher-risk MDS. The study received local Institutional Review Board approval from all participating sites and from the Data Safety and Monitoring Board of the Rare Diseases Branch of the National Institutes of Health, and was registered with http//:clinicaltrials.gov (NCT00352001).