Proteinuria is an important predictor of renal outcome in a variety of renal diseases. Proteinuria exceeding 0.5 g/day is often considered to be a major indication of renal biopsy. In this study, we analyzed the clinical and histopathological data of 58 patients with mild proteinuria of less than or equal to 0.5 g/day. The histopathological diagnosis included 45 cases (77.6%) of mesangial proliferative glomerulonephritis, 4 cases (6.9%) of lupus nephritis, one case of membranoproliferative glomerulonephritis and only 6 cases (10.3%) of minor glomerular abnormality. The percent sclerotic glomeruli exceeded 10% in 17 cases (29.3%) and reached 71.4% in 2 cases. There were no significant differences in histopathological parameters (percent sclerotic glomeruli, tubulointerstitial change, arterio-arterio sclerotic change) between the groups with or without microhematuria. There was a positive correlation between age and percent sclerotic glomeruli. Percent sclerotic glomeruli in our cases were higher than in the healthy population reported by Kaplan et al. and the influence of glomerulonephropathy was obvious. During the follow-up period (mean 19.7 months), one patient progressed to chronic renal failure and 2 patients had increased urinary protein excretion, but the others did not. These results suggest the importance of clarifying the prognosis by renal biopsy even in cases with mild proteinuria.