The non-secretor phenotype was significantly associated with the occurrence of renal scarring among patients with recurrent pyelonephritis. Girls (n = 77) with recurrent pyelonephritis were followed from the first known episode of infection for up to twelve years with repeated radiological investigations. They were divided into two categories: those with renal scars (n = 35) and those who did not develop scars (n = 42). There was a significant over-representation of non-secretors among the patients with scarring, (14/35, 40%) compared to the healthy controls (21.8%, P less than 0.05). The frequency of non-secretors among the girls who did not develop scars in spite of repeated episodes of acute pyelonephritis was not significantly different from the healthy controls (9/42, 21% n.s.). This study provides a basis for analysis of the influence of secretor state on host-parasite interaction in the urinary tract.