P-fimbrial genotypes of Escherichia coli strains and their possible association with urinary tract abnormalities were studied in infants with pyelonephritis. A total of 153 urinary E. coli strains were analyzed by polymerase chain reaction for class I, II, and III alleles of the pyelonephritis-associated adhesin gene papG. Strains with any class II papG alleles were found significantly more often in infants with normal anatomy and function or in infants with clinically insignificant abnormalities than they were in infants with significant abnormalities (90 of 119 vs. 14 of 34 infants; P<. 001). On the other hand, strains without any papG alleles were found significantly more often in infants with major urinary tract abnormalities (11 of 34 vs. 17 of 119 infants; P=.016). Our genotypic findings indicate that, especially in infants with a normal urinary tract, infection is caused by more-virulent E. coli than is present in infants without a normal urinary tract. This virulence could be due to expression of pyelonephritogenic P fimbriae by an infecting E. coli strain.