It has been shown that uncircumcised infants have a higher rate of urinary tract infections in the first few months of life. To investigate further the association between noncircumcision and urinary tract infections, a case-control study was performed of infant boys who had a catheterized or suprapubic urine culture done as part of an acute illness visit. Cases (n = 36) were those infants who had a positive culture (greater than 105 organisms per milliliter); controls (n = 76) had a negative culture (less than 103 organisms per milliliter). There were no significant differences found in the two groups in age, ethnic group, and type of medical insurance. All of the cases were uncircumcised, vs 32% of controls. The data were analyzed separately by age, ethnic group, type of insurance, and method of culture, and in all groups the cases were significantly more likely to be uncircumcised. Of the 31 cases who underwent roentgenographic investigations, 8 had abnormal findings. Noncircumcision seems to be a highly significant risk factor for urinary tract infection in infants up to 12 months of age, affects infants regardless of race and socioeconomic status, and is associated with anatomic abnormalities in 26% of cases.