Experimental evidence suggests a role for obesity in the formation and progression of some glomerular lesions, but data for human glomerulonephritis are lacking. In a cohort of 162 incident patients with biopsy-proven immunoglobulin A (IgA) nephropathy, we assessed whether the presence of an elevated body mass index (BMI >/= 25 kg/m(2)) at the time of the first renal biopsy (RB1) correlated with clinical data at RB1 (24-hour proteinuria, arterial hypertension, and renal function), pathological data (global optical score [GOS] with detailed pathological indices), and clinical progression to both arterial hypertension and chronic renal failure (CRF). In both univariate and multivariate analyses, the presence of an elevated BMI at RB1 was significantly associated with the severity of pathological renal lesions (GOS and vascular, tubular, and interstitial indices). Hypertension-free survival was significantly less in overweight patients (P: < 0.0001) compared with those with normal weight. In a Cox regression analysis for hypertension-free survival including 24-hour proteinuria greater than 1 g, GOS, and metabolic parameters, only elevated BMI and GOS were independent factors for the development of arterial hypertension. CRF-free survival was also significantly less in patients with an excessive BMI. In a multivariate Cox regression analysis for CRF-free survival, hypertension, GOS, and BMI at RB1 were independent risk factors for CRF. In IgA nephropathy, excessive body weight and/or BMI are underestimated predictive factors for the development of arterial hypertension and, ultimately, CRF.