A prospective long-term study was conducted of 235 young to middle-aged women with past histories of and/or active infections of the urinary tract. The present report defines certain characteristics of the population and of the 1,018 confirmed infections of the urinary tract that occurred during observation periods ranging from 1.1 to 19.4 years (mean, 7.4 yr). The population could be divided into 3 groups: women with confirmed infections who did (Group 1, n = 19) or did not (Group 2, n = 191) receive courses of antimicrobial prophylaxis and those who did not have confirmed infections (Group 3, n = 25). The 3 groups differed significantly only in their mean infection rates per year (3.1, 0.8, and 0.0, respectively). The number of infections among individual women ranged from 0 to 42. The patterns of recurrent infections among individual women ranged widely and were not predictable. Clusters of infections occurred in 45.7% of the women and ranged in size from 2 to 12 infections per cluster. The proportions of different infecting organisms and clinical syndromes were similar to those reported by others. accounted for the great majority of infections. Eighty-five percent of the isolates of tested were serologically classified. A total of 50 different O groups were identified. Three (O4, O6, and O75) of the 131 potential O groups accounted for 49% of the isolates that could be classified. In each of the 4 clinical syndromes, these same 3 O groups were also the most frequently identified. The ability to classify serologically the infecting isolates of permitted a more complete definition of the complexity of patterns of recurrent infections among individual women. The findings in the present study are compared with those in the literature.