In a long-term investigation in children and adult patients with urinary tract infection, 1403 Escherichia coli strains were serotyped and 266 strains were subjected to serological investigation. The most frequently occurring O groups were O6, O8, O2, O18, O4, O75, O1, O22, O7 and O9. L type antigens were predominant among the K antigens. Infants in whom pyelonephritis manifested itself for the first time exhibited an above average rate of infection with O2, O4 and O75. A reinfection rate of 84 to 91% was observed in children in follow-up investigations.