IgA nephropathy (IgA N) is the most common type of primary glomerulonephritis (27.5%) in South Korea and leads to renal failure in a significant number of cases. To evaluate the possible prognosticators of this disease, renal biopsy material from 142 Korean patients with IgA N was studied by light-, electron- and immunofluorescent microscopy, and a clinicopathologic correlation was made. Modified classification of Meadow et al.  for Henoch-Schoenlein nephritis was adopted for the histologic grading of glomerular lesions. Twenty-three biopsies (16.2%) exhibited histologic grades IV and V lesions in association with high levels of proteinuria, serum creatinine and blood pressure and a low frequency of gross hematuria when compared to the remaining 119 biopsies with histologic grades I to III lesions. Ninety-one patients were followed for one to 6.5 years (mean, 3.4 years). Seventeen patients (18.7%) had chronic renal insufficiency, of whom eleven eventually showed endstage renal failure. More than 70% of the patients with histologic grades IV and V exhibited progressive renal disease, whereas patients with grades I to III lesions had a benign course (p less than 0.0005). These results suggest that histologic grading may be the best index to predict the present state or the subsequent progression of the lesion in IgA N.