We correlated the height of fever with underlying infectious etiology in 683 consecutive febrile infants aged four to eight weeks who received outpatient evaluation for sepsis during a five-year period. The relative number of infants with fever was inversely proportional to fever height, as 51% had a temperature 38.1-38.9 degrees C, 45% had a temperature 39-39.9 degrees C, and 4% had a temperature greater than or equal to 40 degrees C [hyperpyrexia]. There were 34 cases of serious bacterial infections [SBI], including 16 cases of urinary tract infection, 8 cases of bacteremia, 6 cases of bacterial meningitis, and 4 cases of Salmonella enteritis. The rate of SBI increased in direct proportion to fever height, being 3.2% in those with a temperature 38.1-38.9 degrees C, 5.2% in those with a temperature 39-39.9 degrees C, and 26% in those with a temperature greater than or equal to 40 degrees C. The 6.8% rate of SBI in those with fever greater than or equal to 39 degrees C was significantly greater than the 3.2% rate in those with fever less than 39 degrees C [p less than 0.035]; and the 26% rate of SBI in those with hyperpyrexia was significantly greater than the 4.1% rate in those with fever less than 40 degrees C [p less than 0.000004]. In identifying those with SBI, the presence of hyperpyrexia had a sensitivity of 21%, specificity of 97%, positive-predictive value of 25%, and negative-predictive value of 96%.(ABSTRACT TRUNCATED AT 250 WORDS)