PURPOSE: A phase I trial of intrathecal (IT) mafosfamide was performed to determine the optimal dose, dose-limiting toxicities, and incidence and severity of other toxicities when administered in association with concomitant multiagent systemic chemotherapy to children younger than 3 years with newly diagnosed embryonal tumors. PATIENTS AND METHODS: Twenty-five assessable patients received IT mafosfamide at one of six dose levels ranging from 5 mg to 17 mg. Patients were premedicated with dexamethasone (0.15 mg/kg) and morphine (0.1 mg/kg) before receiving IT mafosfamide. Serial samples of CSF for pharmacokinetic studies were obtained in a subset of patients with Ommaya reservoirs. RESULTS: Irritability, presumably secondary to pain or headache during mafosfamide administration, was dose limiting in two of three patients at the 17-mg dose level. The maximum-tolerated dose of IT mafosfamide following premedication with dexamethasone and morphine was 14 mg. CONCLUSION: The maximum tolerated dose and recommended phase II dose of IT mafosfamide in patients younger than 3 years with newly diagnosed embryonal CNS tumors is 14 mg. A trial to assess the efficacy of regional therapy with IT mafosfamide administered with intensive systemic chemotherapy in children younger than 3 years with primary intracranial embryonal tumors is now in progress.