A phase I study of temozolomide administered orally once a day, on 5 consecutive days, between 500 and 1200 mg m(-2) per 28-day cycle was performed. Children were stratified according to prior craniospinal irradiation or nitrosourea therapy. Sixteen of 20 patients who had not received prior craniospinal irradiation or nitrosourea therapy were evaluable. Myelosuppression was dose limiting, with Common Toxicity Criteria (CTC) grade 4 thrombocytopenia occurring in one of six patients receiving 1000 mg m(-2) per cycle, and two of four patients treated at 1200 mg m(-2) per cycle. Therefore, the maximum-tolerated dose (MTD) was 1000 mg m(-2) per cycle. The MTD was not defined for children with prior craniospinal irradiation because of poor recruitment. Plasma pharmacokinetic analyses showed temozolomide to be rapidly absorbed and eliminated, with linear increases in peak plasma concentrations and systemic exposure with increasing dose. Responses (CR and PR) were seen in two out of five patients with high-grade astrocytomas, and one patient had stable disease. One of ten patients with diffuse intrinsic brain stem glioma achieved a long-term partial response, and a further two patients had stable disease. Therefore, the dose recommended for phase II studies in patients who have not received prior craniospinal irradiation or nitrosoureas is 1000 mg m(-2) per cycle. Further evaluation in diffuse intrinsic brain stem gliomas and other high-grade astrocytomas is warranted.