We present a case of IgA nephropathy (IgAGN) which developed rapidly progressive glomerulonephritis and showed marked clinical improvement with treatment. The patient was a 7-year-old boy who initially presented with acute nephritic syndrome with hypocomplementemia. Although the renal function improved with normalization of the serum complement level, it deteriorated again progressively. The first renal biopsy revealed cellular crescents in about 70 percent of 43 glomeruli. Immunofluorescent microscopy demonstrated deposits of IgA, C3 and IgG in the mesangium; they were also deposited along the glomerular capillary walls. He was treated with plasma exchange associated with hemodialysis and methylprednisolone pulse therapy, followed by oral administration of prednisolone, cyclophosphamide and warfarin. Renal function recovered to the normal range about two months after the initiation of treatment. The second biopsy demonstrated a marked decrease in histological activity. In this case, transient hypocomplementemia at onset may indicate that acute glomerulonephritis caused exacerbation of clinically silent IgAGN. Aggressive therapy may be effective in patients with rapidly progressive IgAGN if treated at an early stage.