Purpose: To conduct a Phase I trial to determine the maximally tolerated dose (MTD) of tipifarnib in combination with conventional three-dimensional conformal radiotherapy (RT) for patients with glioblastoma multiforme. Methods and Materials: After resection or biopsy, tipifarnib was given 1 week before and then continuously during RT (60 Gy), followed by adjuvant administration until progression. The tipifarnib dose during RT was escalated in cohorts of 3 starting at 200 mg/day. Results: Thirteen patients were enrolled, and 12 were evaluable for MTD. Of these patients, 7 had undergone biopsy, 4 had partial resection, and 1 had gross total resection. No dose-limiting toxicity (DLT) was observed during the concomitant treatment at 200 mg. All 3 patients at 300 mg experienced DLT during the concomitant treatment: 1 with sudden death and 2 with acute pneumonitis. The MTD was reached at 300 mg. The adjuvant treatment was suppressed from the protocol after a case of pneumonitis during this treatment. Six additional patients were included at 200 mg/day of the new protocol, confirming the safety of this treatment. Of the 9 evaluable patients, 1 had partial response, 4 had stable disease, and 3 had rapid progression; the patient with gross total resection was relapse-free after 21 months. Median survival of the evaluable patients was 12 months (range, 5.2–21 months). Conclusion: Tipifarnib (200 mg/day) concurrent with standard radiotherapy is well tolerated in patients with glioblastoma. Preliminary efficacy results are encouraging. Purpose: To conduct a Phase I trial to determine the maximally tolerated dose (MTD) of tipifarnib in combination with conventional three-dimensional conformal radiotherapy (RT) for patients with glioblastoma multiforme. Methods and Materials: After resection or biopsy, tipifarnib was given 1 week before and then continuously during RT (60 Gy), followed by adjuvant administration until progression. The tipifarnib dose during RT was escalated in cohorts of 3 starting at 200 mg/day. Results: Thi...