Although there have been many reports on clinicopathological studies of immunoglobulin A nephropathy (IgAN), reliable outcome predictors are still lacking. We therefore assessed maximal glomerular diameter (Max GD), an indicator of glomerular size, as a predictor of the long-term evolution of renal histopathology. Forty-three adult patients, diagnosed with IgAN, who had estimated glomerular filtration rate (eGFR) ≥50 mL/min/1.73 m(2), were enrolled in this study. Prognostic variables for renal survival were examined by using the multivariate Cox proportional hazards method. The optimal cut-off value of Max GD was 242.3 μm (AUC = 0.78, sensitivity = 62.5%, specificity = 81.5%) by using receiver operating characteristics analysis. In order to assess the characteristics of glomerular hypertrophy, we divided the cases into two groups according to the Max GD value (Group A, ≥242 μm; Group B, <242 μm). Renal survival was also assessed by Kaplan-Meier curves with the log-rank test. The Max GD was significantly correlated with age, body mass index and serum triglyceride levels at the time of renal biopsy. During the 10-year follow-up period, the Max GD was significantly correlated with eGFR decline per year, and proteinuria, but not with hematuria. A multiple regression analysis by the Cox method adjusted for age, sex and eGFR showed that the Max GD values were significantly associated with a 1.5-fold increase in serum creatinine (Cr) values (hazard ratio = 1.04, P = 0.03). Renal function in 66.7% of the patients whose Max GD was ≥242 μm had at least a 1.5-fold increase in their serum Cr value at the 10-year follow-up examination (log-rank, P = 0.003). The results of this study suggest that Max GD is a simple quantitative prognostic indicator of the disease progression in IgAN patients.