Guillain-Barré syndrome appeared in two patients following autologous bone marrow transplantation for metastatic breast cancer. In one case, the patient responded to plasmapheresis and became ambulatory. In the second case, the patient died of an unrelated complication: intracerebral hemorrhage. Although Guillain-Barré syndrome is associated with hematologic malignancies, it has not been previously reported in patients with solid tumors undergoing autologous bone marrow transplantation. Because both patients were in remission at the time of the autograft, Guillain-Barré syndrome may emerge as a rare complication of altered immune function and/or viral infection after marrow transplantation.