A retrospective study of 166 patients with IgA nephropathy was undertaken to clarify possible correlations between clinical and histological features, and the severity and prognosis of the disease. At the time of biopsy, impaired renal function, with creatinine clearance (C<sub>cr</sub>) below 90 ml/min was found in 61 cases. At the final examination, after a mean follow-up period of 34 months, 82 patients had impaired renal function, 12 of these patients went into terminal renal failure requiring hemodialysis treatment. The presence of proteinuria of more than 1.0 g/day was closely correlated with impairment of renal function both at the time of biopsy and at the final observation. An unfavorable outcome was also anticipated in the presence of hypertension. In contrast, microhematuria, macrohematuria or high serum IgA levels did not appear to be related to the outcome. Histologically, sclerotic lesions such as mesangial or global sclerosis, interstitial fibrosis and tubular atrophy, and some active changes such as mesangial hypercellularity and tuft adhesion were more frequent and severe in patients with impaired renal function. Impressive localization of IgA and C3 in the mesangium as well as in capillary loops was observed more often in these patients. These results clearly indicate that IgA nephropathy may follow a slowly progressive course in about half of the patients, and that marked proteinuria and severe histological changes appear to correlate closely with an unfavorable course.