• Forty-three patients and 70 controls were enrolled in an open study to evaluate the role of bacterial virulence factors and Tamm-Horsfall protein (THP) in the pathogenesis of urinary tract infection (UTI) due to Escherichia coli. The ages of the patients and controls ranged from 5 days to 6 months. Nine patients had upper UTIs and 33 patients had lower UTIs, based on bacteremia and radiographic evidence. There were no differences between patients and controls in the percentages of strains showing hemolytic activity of E coli strains isolated from periurethral and rectal cultures. Urinary E coli strains recovered from patients showed a significantly higher percentage of mannose-resistant hemagglutination, greater P and X fimbriation, more type 1 pill, and the presence of mannose-sensitive and mannose-resistant hemagglutination when compared with E coli strains recovered from the controls. The O but not the K pyelonephritogenic serotypes were more commonly recovered from patients than from controls. The mean THP concentrations in urine (micrograms of THP per milligrams of creatinine) were 29.9 and 91.4 μg/mg for patients and controls, respectively. With the exception of hemagglutination properties and certain pyelonephritogenic O serotypes, other virulence factors of E coli could not be defined in our Infant population. The role of THP, if any, in the pathogenesis of UTI remains unresolved.