Summary 1. The material consists of 5% consecutive cases of primary (first onset) urinary tract infections appearing from birth up to 16 years of age and which were examined and treated at the Childrens' Hospital in Goteborg. The infections occurred during a seven‐year period within a defined population. The circumstances under which the study was conducted suggest that most symptomatic infections occurring during the study period and for whom the parents sought medical advice, were 2. The total morbidity risk at I 1 years of age of symptomatic UTI was 3.0% for girlsand 1.1 % for boys. These are minimum figures. The morbidity risk is highest during the first month of life and then decreases, more rapidly in boys than in girls. Possible interpretations of the reason for decreasing risk with increasing age of falling ill with a first infection are suggested. The male/female ratio starts at 2.5: 1 during the first month and then successively changes to 1:20.There was no seasonal variation of the time of onset in either sex. 3. Presentation with fever was most common in the first year, after which it slowly decreased. Failure to thrive was a rare symptom. Certain pther age and sex differences in presenting symptoms were recorded. Most infections within the first year of life probablyinvolved the renal parenchyma. 4. The etiology varied with age and sex. If infections reach the urinary tract by theascending route, this could indicate differences in the environmental conditions in the periurethral region and may be a clue to a better understanding of the pathogenesis. 5. Obstructive malformations were found in 10% of boys and 1–2% of girls, and cannotexplain the high frequency of early infantile infections in either sex. 6. Narrowing of the bladder neck was common in males during the first year of life, the frequency declining with age. It disappeared spontaneously during follow‐up of individual cases, and was not regarded as an obstructive malformation. 7. Duplication of the collecting system was seen in 10 % of girls and in 5 % in boys, which is more than expected. The cause and nature of the association between infection and duplication are not known. 8. In 13 % of boys and 4.5 % of girls a reduction of the renal parenchyma was seen either at the first investigation or developed later, probably owing to infection. AlthoughUTI was more frequent in females than in males, the total number of patients with parenchymal damage was equal in both sexes, even during childhood. In boys, the kidneys might bemore vulnerable than in girls. 9. The immediate cure rate after 10 days' therapy was 97%. Recurrences were usually reinfections. 10. Recurrent infections were often difficult to diagnose. Pyuriaand symptoms of UTI were associated with insignificant bacteriuria in 10 % (30 of 300) of suspected recurrences. 11. Susceptibility to recurrence was studied in relation to various parameters. Girlswere at greater risk than boys, and the risk was in both sexes greatest during the first 2–3 months after a previous infection. Boys rarely had a recurrence more than one year after the primary infection, while girls often continued to have recurrences for many years. The risk seemed to increase with the increasing number of previous recurrences. No correlation was found between the frequency of recurrences and age at onset of primary infection or with symptomatology, etiology, reflux, or duration of therapy. 12. The risk of recurrence was many times greater in a patient who had had one infection than in a previously healthy patient. The possibility that the degree of susceptibility to UTI might depend, to some extent, on genetic factors should be examined.

EPIDEMIOLOGY OF SYMPTOMATIC URINARY TRACT INFECTION IN CHILDHOOD