Urinary concentrations of interleukin-1 alpha (IL-1 alpha) and interleukin-1 receptor antagonist (IL-lra) standardized to urinary creatinine concentrations were studied. The median standardized IL-1 alpha creatinine quotient in children with first-time acute pyelonephritis was 3.6 pg/mumol, but was nondetectable in children with recurrent pyelonephritis, children with non-renal febrile conditions and children convalescent after acute pyelonephritis (p < 0.05-0.01). IL-lra levels were also significantly higher in children with acute first-time pyelonephritis (median of 239 pg/mumol) compared to these three groups of children (p < 0.01-0.001). The highest urinary IL-lra levels, however, were found in the healthy controls (median value 1.019; p < 0.001). Both cytokines were higher among children younger than one year compared to older children. The acute IL-1 alpha creatinine quotients were lowest in children with uptake defects on 99mTC-dimercaptosuccinic acid (DMSA) scintigraphy both during the acute infection (reflecting the acute inflammation) (p < 0.001) and 1 year after the acute infection (reflecting permanent kidney scarring) (p < 0.001). In conclusion, persisting high urinary levels of IL-1 alpha were associated with less renal inflammation and scarring.