A retrospective study was performed for all patients diagnosed with haemorrhagic shock and encephalopathy syndrome (HSES) over an 11 year period (1984-94). Soroka University Medical Centre is the only medical facility in the southern Negev region of Israel serving a population of about 400,000 residents, consisting primarily of two ethnic populations, Jews and Bedouins. Twenty patients, 17 Bedouin and three Jews, were diagnosed with HSES. The annual incidence of HSES for infants under the age of 1 year was 5:10,000 for Bedouins and 0.6:10,000 for Jews. Patients ranged in age from 6 to 32 weeks and arrived at the hospital late at night or early morning (2:00 am to 11:00 am), during the winter or early spring (November to April). All were healthy before admission, with short prodromal symptoms of upper respiratory tract or gastrointestinal infection noted in 10 cases. Most infants had markedly high body temperature on arrival. A history of overwrapping and/or excessive heating was obtained in four of 20 infants. Bacteriological and virological cultures were negative in all infants. One infant died and neurological sequelae were observed in all survivors. The high prevalence of hyperpyrexia during sleep in the presence of negative microbiological results with no evidence of excessive heating, and the high incidence of HSES among a closed and culturally isolated society known to have a high incidence of congenital malformations, may support previous assumptions that HSES results from hyperpyrexia, originating in most cases from a 'physiological' heat induced trigger, which starts and peaks during the night in previously healthy infants who are genetically susceptible.