PURPOSE: To define the maximum tolerated dose (MTD) of lenalidomide, an analogue of thalidomide with enhanced immunomodulatory and antiangiogenic properties and a more favorable toxicity profile, in patients with newly diagnosed glioblastoma multiforme (GBM) when given concurrently with radiotherapy. PATIENTS AND METHODS: Patients with newly diagnosed GBM received radiotherapy concurrently with lenalidomide given for 3 weeks followed by a 1-week rest period and continued lenalidomide until tumor progression or unacceptable toxicity. Dose escalation occurred in groups of 6. Determination of the MTD was based on toxicities during the first 12 weeks of therapy. The primary endpoint was toxicity. RESULTS: Twenty-three patients were enrolled, of whom 20 were treated and evaluable for both toxicity and tumor response and 2 were evaluable for toxicity only. Common toxicities included venous thromboembolic disease, fatigue, and nausea. Dose-limiting toxicities were eosinophilic pneumonitis and transaminase elevations. The MTD for lenalidomide was determined to be 15 mg/m(2)/d. CONCLUSION: The recommended dose for lenalidomide with radiotherapy is 15 mg/m(2)/d for 3 weeks followed by a 1-week rest period. Venous thromboembolic complications occurred in 4 patients, and prophylactic anticoagulation should be considered.