The occurrence of Escherichia coli possessing P blood-group-specific adhesins (P-fimbriae) was examined in 97 children with urinary tract infections and 82 healthy controls. P-fimbriae were present in 91% (33/35) of the urinary strains causing acute pyelonephritis. Among strains causing cystitis and asymptomatic bacteriuria P-fimbriae were found in 19% and 14% of cases, respectively. Only 7% of faecal isolates from healthy controls carried P-fimbriae. The results were similar in three different studies. In most of the children with acute pyelonephritis the urinary pathogen was the predominant E. coli strain of the periurethral and faecal flora.