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We describe a 39-year-old Japanese man with post-streptococcal acute glomerulonephritis (PSAGN) super-imposed on long-term immunoglobulin A nephropathy (IgA-N). The histological findings of the first renal biopsy, done at 21 years of age, revealed mild mesangial proliferative glomerulonephritis with mesangial IgA deposition. Nineteen years later, acute nephritic syndrome with hypocomplementemia and an increasing anti-streptolysin O (ASO) titer developed 2 weeks after the onset of an upper respiratory infection. A second renal biopsy revealed severe segmental endocapillary proliferative and exudative glomerulonephritis, with fibrocellular crescents in about 40% of the glomeruli. Immunofluorescence showed that more C3 than IgA was deposited in the mesangium and that the IgA deposits had decreased. Electron microscopy revealed “hump” electron-dense deposits on the epithelial side of the glomerular basement membrane. These features were consistent with PSAGN superimposed on IgA-N. After 2 weeks of observation, blood pressure, C3 level, and ASO titer had returned to normal, although the persisting nephritic syndrome necessitated steroid therapy. Six months after the onset of the acute nephritic syndrome, the patient remained asymptomatic, except for microhematuria. [ABSTRACT FROM AUTHOR]