One hundred ninety-eight children were entered on POG 8633, "Prolonged Postoperative Chemotherapy and Delayed Radiation for Children <3 years of age with Malignant Brain Tumors" (1986-1990). Thirteen manifested supratentorial nonpineoblastoma primitive neuroectodermal tumors, making this the second most common supratentorial tumor in the study. Symptoms and signs included seizures, nausea, vomiting, lethargy, irritability, headache, focal motor weakness, and increased head circumference. Twelve of 13 had symptoms for less than 1 month before diagnosis. The average tumor size was 5.96 cm (+/- 0.37) x 5 cm (+/- 0.28) x 5.15 cm (+/- 0.31). Eight tumors were predominantly hemispheral, and five were midline. Computed tomographic scans on nine patients revealed tumor hyperdensity (nine), midline shift (eight), hydrocephalus (seven), cysts (six), well-defined borders (five), and calcification (four). No patients exhibited peritumoral edema. Contrast enhancement, primarily heterogeneous, was present in all patients. Magnetic resonance imaging appearance of the tumor (six patients) demonstrated midline shift (four), well-defined margins (four), necrosis (two), cysts (three), and hemorrhage (two). No peritumoral edema was present. Most enhanced heterogeneously. The diagnosis of supratentorial nonpineoblastoma primitive neuroectodermal tumors should be suspected when a large, sharply marginated, hyperdense supratentorial mass is observed in a young child, particularly when no peritumoral edema is present.