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From 1976 to December 1984 IgA nephropathy (IgAN) was diagnosed in 40 patients at the Nephrology Division of the Hospital das Clínicas, São Paulo University. In order to estimate the incidence of the disease in our city, we reviewed the kidney biopsies of the year 1984: IgAN constituted 6.5% of all biopsies of primary glomerular nephropathies. Of the 40 patients 67.5% were males and 32.5% females. Their age varied between 4 and 49 years and 72.5% were in the 2nd and 3rd decades of life at the beginning of the disease. From the racial standpoint, 82.5% of the patients were white, 10% were yellow and 7.5% black. The most frequent clinical manifestation, presented by 62.5% of patients, was macroscopic hematuria in close association with an acute infectious process. In the remaining 37.5% of cases the clinical picture of the disease was very variable, and did not evoke the diagnosis of IgAN. The initial laboratory investigation showed proteinuria in 92.5% of patients; it was under 1g/24h in 40%, it ranged from 1 to 3g/day in 37.5% and was over 3.5g/24h in 15% of them. Hematuria was present in all 40 cases and the red cells displayed moderate dismorphism. Glomerular filtration rate, evaluated by creatinine clearance in 39 patients, was over 90 ml/min/1.73m2 in 70%, ranged between 50 and 89 ml/min/1.73m2 in 7.5% and was under 25 ml/min/1.73m2 in 20% of them. Total complement CH50 was found normal in 79.4% and was slightly reduced in 20.6%. The C3 fraction showed slight reduction in 9.5% and C4 was found normal in all instances. Serum IgA was increased in 21% of the cases. All patients were submitted to percutaneous renal biopsy. Proliferative mesangial lesions were found in 82.5% of cases; they were focal in 42.5% and diffuse in 40%. In 15% of the patients, the glomeruli were entirely normal on light microscopy. In one case they were crescent-shaped, exhibiting 82% of the glomeruli this form. The immunofluorescence findings were quite uniform, with granular deposits occupying only the mesangium, with a global and diffuse distribution in all glomeruli. IgA was the dominant immunoglobulin deposited in the 40 cases and C3 was also found in the same localization and distribution in all biopsies, generally with a lesser intensity than IgA. IgG was present in 85% of cases and IgM in 32.5% of the biopsies, in the majority of them with the intensity of traces.(ABSTRACT TRUNCATED AT 400 WORDS)