In order to improve the diagnosis of thin basement membrane nephropathy, we observed the ultrastructure of renal biopsy specimens from five cases with thin basement membrane nephropathy, and selectively measured the thickness of the basement membrane. The result showed: 1. The thickness of the basement membrane of five patients was less than 250 nm. 2. The thickness of the basement membrane was positively related to the course of disease (r = 0.65), While negatively related to the grade of haematuria (r = -0.39). The results suggest that hypoplasia may be the cause of extensively thinning of the basement membrane.