Purpose: To evaluate the toxicity and maximum tolerated dose (MTD) of erlotinib plus radiation therapy (RT) in patients with glioblastoma multiforme (GBM) in a multicenter phase I trial. Methods and Materials: Patients were stratified on the basis of the use of enzyme-inducing anticonvulsants (EIACs). After resection or biopsy, patients were treated with erlotinib for 1 week before concurrent erlotinib and 6 weeks (60 Gy) of RT and maintained on erlotinib until progression. The erlotinib dose was escalated in cohorts of 3 starting at 100 mg/day. Results: Twenty patients were enrolled and 19 were evaluable for the MTD and efficacy endpoints. Of these patients, 14 were males and 5 were females, with a median age of 54 years. Seven had undergone biopsy only, 5 had subtotal resections, and 7 had gross total resections. The highest dose level was 150 mg/day erlotinib for patients not on EIACs (Group 1) and 200 mg/day for patients on EIACs (Group 2). MTD was not reached in either group. In Group 1 at 100 mg (n = 6) and at 150 mg (n = 4), only 1 dose-limiting toxicity (DLT) occurred (stomatitis at 100 mg). No DLTs have occurred in Group 2 at 100 mg (n = 3), 150 mg (n = 3), and 200 mg (n = 3). With a median follow-up of 52 weeks, progression was documented in 16 patients and 13 deaths occurred. Median time to progression was 26 weeks, and median survival was 55 weeks. Conclusion: Toxicity is acceptable at the current doses of erlotinib plus RT. The study was modified to include concurrent and adjuvant temozolomide, and accrual is in progress. © 2006 Elsevier Inc. All rights reserved.