196 episodes of urinary tract infection in children were analysed. 74 were classified as pyelonephritic (PN), 61 as cystitis (C), and 61 as asymptomatic bacteriuria (ABU) on the basis of three clinical signs (elevated temperature, erythrocyte sedimentation rate, and/or white blood cell count). The frequency of P-fimbriae was found high in PN (77%), and significantly lower in C (23%), ABU (20%) and among fecal strains (16%). The common, type 1 fimbriae were also more frequent in PN (92%) than in the other groups (84-76%), whereas other, so-called X-fimbriae, were relatively rare in all the patient groups (15-6%). P-fimbriation was not significantly associated with the presence or absence of reflux or obstructive anomalies. By contrast, the frequency of P-fimbriation increased with increasing severity of the clinical symptoms of pyelonephritis (95% in episodes of elevated temperature, erythrocyte sedimentation rate and white blood cell count).