The diarylsulfonylureas have shown promise in xenograft models of childhood cancer. Sulofenur has been evaluated in phase I and II trials in adults with a variety of solid tumors, but the toxicity and maximum tolerated dose of sulofenur in children and adolescents have not been determined. In a phase I study, sulofenur was administered to 13 patients with refractory pediatric malignant solid tumors. Daily dosages of 640, 800, and 960 mg/M2 in two divided oral doses were given for 5 consecutive days each week for 3 weeks. The primary and dose-limiting toxicity was methemoglobinemia, which occurred at all dose levels and required transfusions of packed red blood cells, administration of methylene blue, or both. Anemia and, less frequently, leukopenia and thrombocytopenia were also observed. A maximum tolerated daily dosage was not defined, as methemoglobinemia was noted with each dosage level. There were no measurable tumor responses. The toxicity of this agent makes it unattractive for further investigation in pediatric patients.