Two populations were analyzed prospectively after a first episode of urinary tract infection. Asymptomatic infants were screened at 2 weeks and at 3 and 10 months of age, and bacteriuria was confirmed by bladder puncture. Infants with febrile urinary tract infection were enrolled during the same study period. The inflammatory response was characterized by the presence of fever, serum C-reactive protein, microsedimentation rate, urinary leukocyte excretion, and width of the ureters. The bacteria were defined for O:K:H serotype, hemolysin production, resistance to the bactericidal effect of serum, attachment to uroepithelial cells, and specificity for the globoseries of glycolipid receptors. In agreement with previous studies, the frequency of increased inflammatory signs and of attaching Escherichia coli was significantly higher in infants with febrile urinary tract infection than in the screening group. Within both patient groups, however, children infected with attaching E. coli strains had significantly more inflammatory signs. The results suggest that adherence facilitates the presentation of bacterial components capable of causing inflammation in the tissues in the urinary tract.