Bacterial attachment to urinary-tract epithelium is important in the pathogenesis of urinary-tract infection. Most pyelonephritogenic Escherichia coli bind specifically to epithelial-cell receptors, which are glycolipids of the globoseries and also antigens in the P blood-group system. Among 36 girls with recurrent pyelonephritis who did not have vesicoureteral reflux, we found that attaching bacteria were common and the P1 blood-group phenotype was present in 97 per cent, as compared with 75 per cent of 84 age-matched children without urinary-tract infection (P less than 0.01). In 32 girls with recurrent pyelonephritis who had reflux, attaching bacteria were rare, and the frequency of the P1 phenotype was not significantly higher than in controls (82 per cent, P greater than 0.05). In the group of patients with the P1 phenotype, 68 per cent of the urinary bacterial isolates from those without reflux, but only 25 per cent of isolates from those with reflux, bound to globotetraosylceramide, as determined by a receptor-coating technique (P less than 0.001). Our data suggest that, in the absence of reflux, the P1 blood group contributes to susceptibility to recurrent pyelonephritis due to bacteria that bind to the glycolipid receptors of the globoseries. In the presence of reflux, uroepithelial attachment does not seem to confer an advantage to bacteria that infect the kidney.