In a one-year prospective study we assessed the incidence of Reye's syndrome in children presenting with the acute onset of vomiting after a prodromal upper-respiratory-tract infection or varicella, and with serum alanine or aspartate aminotransferase levels at least three times higher than normal, and a paucity of neurologic findings. Of 25 patients meeting the above criteria, 19 had liver biopsies yielding adequate tissue for diagnostic purposes. Biopsy specimens from 14 of these 19 patients (74 per cent) were diagnostic of Reye's syndrome, according to rigorous light-microscopical, histochemical, and ultrastructural criteria. None of the biopsy specimens contained evidence of other acute pathologic processes, including hepatitis. A wide spectrum of mitochondrial alterations existed at the ultrastructural level, ranging from mild to severe lesions that were indistinguishable from those seen in comatose patients with Reye's syndrome. Our findings suggest that the clinical complex of vomiting, hepatic dysfunction, and minimal neurologic impairment after varicella or an upper-respiratory-tract infection usually represents Reye's syndrome. This syndrome occurs more frequently than previously recognized.