The pathological role of obesity in the progression of glomerular disease has rarely been studied in primary glomerular disease. The aim of this study was to investigate the effect of non-diabetic obesity on the clinicopathological manifestations of IgA nephropathy. Seventy-four patients diagnosed with IgA nephropathy by renal biopsy were retrospectively assigned to two groups according to Japanese obesity criteria: the non-obese group (group N: n=50) with BMI <25 kg/m2 and the obese group (group O: n=24) with BMI ≥25 kg/m2. clinical and pathological data of the patients at renal biopsy were analyzed. In addition, treatment with angiotensin-converting enzyme inhibitor (ACEI) or angiotensin II receptor antagonist (ARB) was followed up for 1 year to assess the treatment effect of proteinuria between the two groups. Urinary protein excretion was significantly increased in the obese group compared with the non-obese group (P<0.05). There was no significant difference in the incidence of hypertension and hyperlipidemia between the two groups. Under light microscopy, the glomerular volume was significantly larger in the obese group (p<0.0001); however, there was no difference in the severity of increased thylakoid matrix and crescent formation between the two groups. On transmission electron microscopy, the glomerular basement membrane (gbm) was significantly thickened in the obese group (p<0.001). 15 of the 61 patients followed for 1 year were treated with acei or arb without steroids. Treatment with acei or arb without steroids was aimed at reducing proteinuria in the obese group, but this change in treatment did not achieve statistical significance. < span="">Glomerular volume was significantly larger in the obese group (p<0.0001); however, the severity of increased thylakoid matrix and crescent formation did not differ between the two groups. On transmission electron microscopy, the glomerular basement membrane (gbm) was significantly thickened in the obese group (p<0.001). 15 of the 61 patients followed for 1 year were treated with acei or arb without steroids. The aim of treatment with acei or arb without steroids was to reduce proteinuria in the obese group, but this therapeutic change did not achieve statistical significance. In patients with IgA nephropathy, obesity leads not only to an increase in glomerular volume but also to changes in the ultrastructure of the glomerular basement membrane (GBM), both of which increase proteinuria in patients.