The medical history of infants and preschool children with their first known episode of pyelonephritis was obtained by questionnaire and compared with that of controls matched for age, sex and living area. Previous infection, medical consultation and antibiotic treatment were registered significantly more often in the patients. Furthermore a high proportion of cases had recently received antibiotic therapy, primarily penicillin V. Urinary tract infections were reported more often among first degree relatives of the cases. Exclusive breast-feeding was significantly shorter in the pyelonephritis patients, according both to the questionnaire and to the records from the Child Health Centers. The two populations were not shown to differ in birth weight or length, neonatal history, number of hospitalizations, previous episodes of fever or frequency of atopic disease in the families.