To find the incidence of bacteremia and serious bacterial infections in febrile children without an apparent focus of infection, we prospectively studied 100 febrile children aged 1 month-3 years with a rectal temperature > or = 39 degrees C. Ten children had a blood culture positive bacteremia and nine had serology positive for bacteremia; 6 had urinary tract infection, 5 otitis media and 8 meningitis. A diagnosis of non bacterial illness was made in 62 patients. Staphylococcus aureus was the most common bacteriologic isolate on blood culture (five) and by serology (eight). TLC > or = 15,000/cu mm m-ESR > or = 25 mm and temperature > or = 39 degrees C had high specificity (95-100%) but low sensitivity for diagnosis of bacteremia.