BACKGROUND: This noninterventional postauthorization safety study assessed the safety and effectiveness of lenalidomide in patients with transfusion-dependent, International Prognostic Scoring System (IPSS) Low- or Intermediate (Int)-1-risk myelodysplastic syndromes (MDS) associated with isolated deletion of 5q (del[5q]) who were treated in routine care. PATIENTS AND METHODS: Eligible adult patients in the lenalidomide cohort had transfusion-dependent, IPSS Low- or Int-1-risk MDS and isolated del(5q) and had received ≥ 1 dose of lenalidomide between 2014 and 2022. The primary endpoint was the 24-month cumulative incidence of acute myeloid leukemia (AML) progression. Overall survival (OS) was estimated by Kaplan-Meier analysis and safety data were collected. RESULTS: In total, 296 patients received ≥ 1 dose of lenalidomide (lenalidomide cohort, safety population) and 277 had received ≥ 1 complete cycle of lenalidomide (primary population). In the safety population, 44.3% of patients completed 3-year follow-up and 55.1% discontinued, with 33.1% discontinuing due to death. In the primary population, 24-month cumulative incidence of AML progression was 12.7% (95% confidence interval, 8.9%-17.1%) and estimated OS probability was 78.3% at 24 months and 63.9% at 36 months. Grade 3/4 treatment-emergent adverse events were experienced by 67.2% of the safety population, and these led to discontinuation in 35.5% of patients. There were no new safety signals. CONCLUSION: These real-world data support the established benefit-risk profile of lenalidomide in transfusion-dependent IPSS Low- or Int-1-risk MDS with isolated del(5q).