Intravenous immunoglobulin is purified, concentrated immunoglobulin G antibodies pooled from human blood donors. The passive transmission of various antibodies from intravenous immunoglobulin has been reported. However, to the best of our knowledge, there are no reports of acquisition of treponemal antibody from immunoglobulin therapy. A woman with a pregnancy complicated by neonatal alloimmune thrombocytopenia was treated with intravenous immunoglobulin to manage her fetal thrombocytopenia. The patient had no history of a syphilis infection. The patient's blood was screened for syphilis antibodies regularly and routinely because she donated platelets for transfusion to her fetus. During her intravenous immunoglobulin treatments, a positive result on a fluorescence antibody absorption test was confirmed, but the result on a rapid plasma reagin test was negative. Eleven weeks after her final dose, results of the fluorescence antibody absorption test were negative, with a negative rapid plasma reagin test result, suggesting passive acquisition of the treponemal antibody. Clinicians and pathologists must be aware of the possible acquisition of this antibody during the treatment and counseling of patients receiving intravenous immunoglobulin.