In idiopathic thrombocytopenic purpura, intravenous immunoglobulin therapy is currently a treatment of choice. It is usually effective with little side-effects. Acute renal failure has been recently identified as a potential, though rare, complication of intravenous immunoglobulins. However, the mechanism remains unclear. A 53 year-old-woman with idiopathic thrombocytopenic purpura developed oliguric renal failure after high doses of intravenous immunoglobulins. Her renal function returned to normal after dialysis and plasmapheresis. This rapid normalization suggested that acute renal failure was functional. Since laboratory tests showed an increase in protidemia concomitant to the immunoglobulin therapy, acute renal failure was likely to be related to hyperosmolar renal damage.