Nine centers collaborated to determine the rate of urinary tract infection in infants with unexplained fever, to determine whether the rate is higher in febrile infants than in asymptomatic infants, and whether the yield justifies urine cultures in febrile infants. Urine cultures were done in 501 infants 0 to 2 years of age. The rate of confirmed urinary tract infections in the 193 febrile infants was 4.1%. All infections were in girls, with a rate of 7.4%. The rate of confirmed urinary tract infections in the 312 asymptomatic infants was 0.3%; again, all infections were in girls, with a rate of 0.7%. The rate in febrile girls was significantly higher than the rate in asymptomatic girls (P less than 0.01). The data support the advisability of culturing the urine of infant girls with unexplained fever.