Immunoglobulin A (IgA) nephropathy progresses without apparent signs. The only available means of knowing the precise incidence of this disease in children is to analyze data from the school urinary screening system. Students between 6 and 15 years old with IgA nephropathy detected by school urinary screening from 1983 to 1999 in Yonago City, Japan, were examined. In addition, two hospitals with pediatric care units administered a questionnaire and patients with pediatric IgA nephropathy detected by means other than the school urinary screening program were recruited. Of 270,902 children examined by the school urinary screening system, 29 demonstrated suspected chronic nephritis on the basis of abnormal urinalysis and underwent renal biopsy procedures. Among the 29 children, 14 cases of IgA nephropathy were confirmed. During the same period, of the children examined by means other than the school screening program, 20 demonstrated suspected renal disease and underwent renal biopsies. Among these 20 children, 6 cases of IgA nephropathy were diagnosed. In all, 20 cases of pediatric IgA nephropathy were detected over the past 17 years. The incidence rate of pediatric IgA nephropathy was 4.5 cases/year per 100,000 children under 15 years. The incidence of pediatric IgA nephropathy is higher than that of idiopathic nephrotic syndrome.