BACKGROUND/AIM: Lenalidomide (LND) is an oral antineoplastic agent used in the treatment of various malignant hematologic diseases, including multiple myeloma. Major adverse events of LND include myelosuppression, pneumonia, and thromboembolism. Thromboembolism is an adverse drug reaction (ADR) associated with poor outcomes, therefore anticoagulants are administered prophylactically. However, LND-induced thromboembolism has not been clearly characterized from clinical trials. The purpose of this study was to evaluate the incidence, timing, and outcome details of thromboembolism caused by LND using the JADER (Japanese Adverse Drug Event Report) database. PATIENTS AND METHODS: ADRs due to LND reported from April 2004 to March 2021 were selected. Data on thromboembolic adverse events were analyzed and relative risks were estimated using reported odds ratios (RORs) and 95% confidence intervals (CIs). In addition, the time of onset and outcome of thromboembolism were analyzed. RESULTS: There were 11,681 adverse events attributed to LND. Of these, 306 were thromboembolisms. The most frequently reported thrombosis with the highest ROR was deep vein thrombosis (DVT) (165 cases, ROR=7.12, 95%CI=6.09-8.33). The median onset of DVT (quartiles, 25-75%) was 80 (28-155) days. The parameter value (β) was 0.87 (0.76-0.99), suggesting the onset of DVT early in treatment. The prognosis of DVT due to LND was recovery and remission in 34% and 43% of patients, respectively, but 7.9% did not recover. CONCLUSION: DVT is the most frequent thromboembolism in LND, and early treatment is important.