To determine whether bacterial colonization is a factor in the pathogenesis of urinary tract infection, we compared the periurethral bacterial flora of uncircumcised versus circumcised boys during the first year of life. Intraurethral and circumferential glans cultures were obtained from 25 circumcised and 25 uncircumcised infants at 2 days, 2 weeks, 2 months, 4 months, 6 months, and 12 months of age. Different children were used at each age (300 total). The types of bacteria and the total and specific colony counts were compared. The results of the glans cultures were similar to those from the urethra. Uncircumcised boys had significantly higher total colony counts (p less than 0.003) at all ages except 12 months. Escherichia coli was present significantly more often (p less than 0.01) in the urethras of uncircumcised boys at 2 weeks, 2 months, 4 months, and 6 months. Gram-negative uropathogenic organisms (Klebsiella-enterobacter, Proteus mirabilis, and Pseudomonas aeruginosa) were cultured more frequently (p less than 0.0005) from the urethras of uncircumcised boys at 2 months, 4 months, and 6 months. The specific colony counts for E. coli and the other uropathogenic organisms were significantly higher (p less than 0.05) at all ages except 12 months. We conclude that during the first 6 months of life, the presence of a foreskin is associated with a greater quantity of periurethral bacteria and a greater likelihood for the presence of, as well as higher concentrations of, potentially uropathogenic organisms.