The actual state of IgA nephropathy in Japan was surveyed throughout the nation by a questionnaire. Five hundred patients were collected from 26 departments of pediatrics and 2, 1 75 from 27 internal medicine. IgA nephropathy accounted for 19.2% of the children and 30.0% of the adults among primary glomerular diseases. Most of the patients, regardless children and adults, were detected by chance proteinuria and/or hematuria. No significant difference of renal glomerular findings were observed morphologically, between the children and the adults. IgA nephropathy was characterized by relatively benign clinical course, namely, 87.2% of the children and 74.6% of the adults had a favorable outcome. Nephrotic syndrome developed in 4.6% of the children and 3.7% of the adults, and hypertension in 4.6% and 10.6% of the children and the adults, respectively. While, the fact should not be neglected that 1.8% of the children and 8.0% of the adults developed renal failure or died.