The Research Group on Progressive Renal Diseases conducted a national survey, in Japan, of cases of primary glomerulonephritis (GN) in 1985 and 1993. The results of the survey, reported here, revealed a high prevalence and relatively poor prognosis for immunoglobulin A nephropathy (IgAN). Using immunofluorescent microscopy, 47.2% of 1,063 patients were diagnosed as having IgAN; 62.8% of patients had diffuse mesangial proliferative GN, and focal mesangial proliferative GN was observed in 23.0%. Nearly 70% of the patients had no clinical symptoms, and the IgAN was detected by routine physical examination. The mean period of observation was 11.8 +/- 6.3 years. Renal survival rates for the 502 cases of IgAN, in which the start of dialysis and renal-related death were end points, were 96%, 85%, 75%, and 61% at 5, 10, 15, and 20 years, respectively, from the time of the detection of the earliest known renal abnormalities. Renal survival rates of patients with diffuse mesangial proliferative GN were 96%, 83%, 75%, and 59% at 5, 10, 15, and 20 years, respectively. At the end of the observation period, 20% of patients had improved, 45.8% showed no change, 13.5% had deteriorated, and 20.4% had renal-related death. The risk factors for renal failure by logistic multivariate analysis were serum creatinine concentration > or =1.4 mg/dL (relative risk, 3.5) and levels of urinary protein > or = +(dipstick) (relative risk, 6.4), determined at the time of biopsy. These parameters can be useful for assessing prognosis during the relatively advanced stages of this disease. It is important to note that a relatively high percentage of patients with IgAN progressed to end-stage renal failure even when their histologic findings comprised only minor glomerular abnormalities or focal proliferative changes.