A 17-year-old male with poststreptococcal acute glomerulonephritis (PSAGN) superimposed on the course of IgA nephropathy is presented. The histological findings of the first renal biopsy showed mild IgA nephropathy with a mesangial deposition of IgA and C3. Eighteen months later, acute nephritic syndrome with hypocomplementemia and rising antihyaluronidase titer occurred 10 days following the onset of an upper respiratory infection. The second renal biopsy revealed severe diffuse endocapillary proliferative and exudative glomerulonephritis with cellular crescents in 70% of the glomeruli. Immunofluorescence showed granular staining of C3 alone along the capillary walls. The pre-existing IgA deposits had disappeared. Typical ‘humps’ were observed by electron microscopy. The symptoms were gradually resolved by intensive steroid and anticoagulant therapy. Five months after the episode of acute nephritic syndrome, the patient was clear of symptoms except for mild proteinuria and hematuria. The third renal biopsy at that time showed morphologic changes similar to those of the first renal biopsy with mild mesangial IgA deposits.