The authors report the cases of five previously well children, aged 8 to 33 months, who were seen over a 14-year period, with admission temperatures in excess of 42.0 degrees C (107.6 degrees F). Four of the patients died. Each child had a similar clinical illness in which the hyperpyrexia played a critical role. Negative blood, cerebrospinal fluid, and stool cultures excluded bacterial sepsis as a possible etiology. This illness is similar, if not identical, to the newly described syndrome of hemorrhagic shock and encephalopathy (HSES) reported in European and American infants.