Intravenous immunoglobulin (IVIg) is a useful tool in the treatment of a variety of neuromuscular disorders. Though IVIg therapy is generally safe, hemolytic anemia is a potentially serious complication that is often overlooked, and is currently not listed in product inserts. We analyzed 45 patients who received IVIg therapy, including 38 consecutive patients who received IVIg over a 13-month period. On 42 patients, direct antiglobulin testing was performed, searching for antibodies to the patients' own blood type. Of these 42 patients, 12 developed passive sensitization with antibodies to their own blood group antigens after receiving IVIg. Of these 12 patients, 11 patients developed hemolysis severe enough to lower the hemoglobin level by at least 1 g/dL. Of these patients, 3 required blood transfusion, and 1 had IVIg therapy truncated because of the hemolysis. Antibodies to blood group antigens are found in all commercial preparations of IVIg. Though most patients do not have clinically significant hemolysis, clinicians should be aware of this potentially serious complication. Careful monitoring of hemoglobin levels during IVIg therapy is recommended.