A case of acute transverse myelitis following primary antiphospholipid syndrome was presented. The patient was 32-year-old Japanese female who was admitted to our hospital with paresthesia and weakness of the right upper extremity in September 1995. Neurological examinations revealed slight weakness and hypesthesia of the right arm and leg, impairment of vibration sense below the right knee, and hyperreflexia on the right side. There was no apparent history of systemic lupus erythematosus or multiple sclerosis. Laboratory investigations disclosed raised anticardiolipine antibody level of the IgG class. A magnetic resonance (MR) imaging of the cervical spine revealed changes in the spinal cord at C3-5. A gradual enlargement of the cord at C3-5 level was revealed on T1-weighted sagittal images; increased signal intensity at these same levels was visualized on T2-weighted images. Following administration of contrast material, an area of increased signal intensity was visualized within the cord extending from the C3/4 level to the C4/5 level in sagittal views and was located in the right lateral and posterior funiculus of the cord in axial views. After the admission, the patient's neurological symptoms worsened, and then we treated her by steroid pulse therapy. After that, the patient's symptoms gradually disappeared and the abnormal signal intensity of the spinal cord on an MR imaging disappeared. No evidence of recurrence was yielded by neurological examinations or neuroimaging studies. Antiphospholipid antibodies should be studied in all patients with transverse myelitis whether known to systemic lupus erythematosus, multiple sclerosis or not.