In order to clarify the minimum requirements for renal biopsy size for a more accurate interpretation of renal biopsy information by light microscopy, we reanalyzed 92 open renal biopsy specimens in 92 patients with IgA nephropathy retrospectively. The mean number of functioning glomeruli per square millimeter in open renal biopsy specimens from 38 patients with a poor outcome was 1.2 ± 0.4. In contrast, the mean number in 54 biopsy specimens from 54 patients with stable conditions for 10 years was 3.0 ± 0.6. Therefore, there was a significant reduction (p < 0.0001) in the numbers of functioning glomeruli in patients with a poor outcome. We found that the most important point in predicting the severity of tissue damage was detection not of the total numbers of glomeruli but of the density of glomeruli in given specimens in patients with IgA nephropathy. A biopsy sample of 1 mm × 7 mm in size of cortical origin is the minimum necessary to confidently interpret biopsy specimens from patients with IgA nephropathy.