To clarify age-related differences in the characteristics of IgA nephropathy, we investigated 117 patients of all ages. The number of patients in the second decade of age was larger than that in the other age decades. There was no difference in sex in all age brackets. About one half of the patients under ten years of age presented as acute nephritis, but their prognosis was favorable. Patients over 10 years, most of whom were detected by chance, tended to have a greater degree of proteinuria, a lesser degree of creatinine clearance, a higher frequency of hypertension, and a higher level of serum cholesterol with age. Although the intensity of mesangial cell proliferation was not changed, the grade of glomerulosclerosis, interstitial change, and arteriosclerosis increased and the prognosis became poor as age advanced. Treatment with corticosteroids and antiplatelet agents was less effective in adults, especially in the older age brackets than in children because the frequency of histologically chronic lesions increased. In these cases, lipid-lowering agents and angiotensin-converting enzyme inhibitors may be helpful in preventing the progression.