Bacterial adherence to carbohydrate receptors on the surface of urothelial cells is important in the pathogenesis of urinary tract infections. Blood group-related antigens, genetically determined carbohydrate structures found on the urothelial cell surface, may influence the availability of these receptors thereby affecting bacterial adherence and the susceptibility to urinary tract infections. Recent evidence indicates that the immunoanatomical distribution of type 1 blood group-related antigens in urothelium is influenced by ABO, Lewis and secretor phenotypes, women with Le(a-b-) and Le(a+b-) blood phenotypes have more than a 3-fold greater risk of recurrent urinary tract infections compared to Le(a-b+) women and epithelial cells from nonsecretors have more bacterial receptors than cells from secretors. To determine the relation between the expression of type 1 blood group-related antigens and urinary tract infections we performed an immunohistochemical analysis using a well characterized panel of monoclonal antibodies on 72 surgical specimens obtained from children who underwent correction of a structural genitourinary anomaly at the University of Rochester Medical Center from December 1983 to May 1988. Of 72 children 48 had a history of at least 1 documented urinary tract infection. The differences in the distribution of children by type 1 blood group immunophenotype in the infection and noninfection groups were highly significant (p = 0.003, Fisher's exact test). There is an increased frequency of urinary tract infections in children with genitourinary structural anomalies whose urothelium reflects the nonsecretor phenotype, that is they have minimal or undetectable levels of ABO and Leb reactivity compared to those with intense ABO and/or Leb immunoreactivity. Of 17 children with minimal or no ABO or Leb immunoreactivity 16 (94.1%) belonged to the infection group. Furthermore, 23 of 24 patients (95.8%) without a history of urinary tract infection expressed intense ABO and/or Leb immunoreactivity. It appears that the type 1 blood group-related antigen profile of urothelium influences susceptibility to urinary tract infection and it may be important in identifying patients who would benefit from prophylactic antibiotic therapy or earlier surgical intervention.