The host-response parameters fever, C-reactive protein (CRP), and erythrocyte sedimentation rate (ESR) are activated in concert by cytokines such as interleukin-6 (IL-6). Il-6 is secreted in response to Escherichia coli infection of the urinary tract. This study tested the hypothesis that the level of fever, CRP, and ESR is coregulated in individual patients. Body temperature, CRP, ESR, pyuria, and renal concentrating capacity were analyzed in 692 children with first-time urinary tract infections. The association of the parameters was evaluated by correlation and multiple regression analysis. The body temperature, CRP, and ESR were significantly correlated (r = .54, .58, and .58; P less than .001), and variation in CRP and ESR explained approximately 40% of the variation in fever. In contrast, the renal concentrating capacity and pyruia were weakly or not at all correlated with the febrile response (r = -.22; P less than .001), and less than 10% of the variation in renal concentrating capacity was explained by the other parameters. The results suggest that fever, CRP, and ESR describe the same aspect of the host response to UTI.