In idiopathic thrombocytopenic purpura, a known immune-mediated disorder, intravenous IgG is the treatment of choice. Success and the lack of side effects of intravenous IgG in the treatment of idiopathic thrombocytopenic purpura have encouraged consideration of its use in the treatment of neurologic disorders of presumed autoimmune pathogenesis. In this report, we describe two patients who developed acute renal failure following intravenous IgG treatment. The first patient had chronic inflammatory demyelinating polyneuropathy and was treated with intravenous IgG instead of prednisone because of preexisting diabetes. The second patient had idiopathic thrombocytopenic purpura and received intravenous IgG treatment as part of standard care. The patient with idiopathic thrombocytopenic purpura had unrelated bilateral high-grade renal artery stenosis. Both patients had a creatinine level of 140 mumol/L (1.6 mg/dL) prior to treatment. Renal biopsies performed during acute renal failure in each patient demonstrated marked swelling and vacuolization of the proximal tubular epithelial cytoplasm typical of high-solute-load-induced damage (similar to that associated with the use of mannitol). This report draws attention to the importance of screening for impaired renal function before intravenous IgG therapy is initiated. The patients we describe received standard doses of intravenous IgG at the recommended infusion rate yet developed oliguric renal failure. Awareness of serious side effects and recognition of predisposing factors provide means of avoiding known life-threatening complications of intravenous IgG therapy.