Myelodysplastic syndrome (MDS) is a disease affecting clinically and cytogenetically diverse groups of patients. Consequently, MDS patients differ in their response to the individual agents used to treat their disease. Lenalidomide, which has been covered by the national health insurance system of Korea since 2019, is used to relieve transfusion-dependent anemia and has been shown to achieve a genetic response in MDS patients, especially those carrying the del(5q) mutation. Although the mechanism of action of lenalidomide is not yet clear, it may block malignant cell proliferation directly by inhibiting haplodeficient phosphatase, but also act indirectly by killing malignant cells through an immunomodulatory effect. In clinical studies, low-risk patients with the del(5q) mutation who were treated with lenalidomide had a hematologic response rate of 55-60% and a median survival of 2-2.5 years. The genetic response rate was 50-73%, and the complete genetic response rate 30-45%. However, in high-risk patients, the response rate was low (20-30%). These results demonstrate the potential utility of lenalidomide as a first-line drug for transfusion-dependent, del(5q), low-risk patients in Korea. (Korean J Med 2020;95:74-77)