A prospective bacteriological study in 50 children with acute pyelonephritis (APN) (32 girls and 18 boys) and 132 children with lower urinary tract infections (LUTI) (89 girls and 43 boys) was conducted from May to December 1993. Infection was defined by Kass' criteria and APN was defined by the clinical findings. C-Reactive Protein (CRP) assay and postcontrast computed tomography in the presence of a doubt concerning the diagnosis. Escherichia coli (EC) was the bacterial species most frequently isolated (76%). A systematic search for fimbriae protein adhesins (group PAP: pyelonephritis associated pil) on the EC was performed by haemagglutination (human group A red blood cells). 64% of EC possessed fimbriae protein adhesions in the APN group versus only 20% in the LUTI group. In children in whom an organic abnormality was demonstrated, the incidence of fimbriae protein-positive EC was 33% while in children with no organic abnormality, particularly without reflux, 89% of EC presented fimbriae protein. A statistically significant difference was demonstrated between these two groups (p < 0.01). The results of this study illustrate the important role of these adhesins in the development of APN. These adhesins facilitate countercurrent ascension of bacteria in the ureter towards the upper urinary tract and can make the bacteria resistant to certain antibiotics. Testing for fimbriae protein can be useful in clinical practice when investigating the aetiology of APN in the absence of demonstrated reflux. A latex test should soon be available to facilitate the detection of fimbriae protein.