Background: Anticardiolipin antibodies (ACL-Ab) are associated with various neurologic syndromes, but idiopathic intracranial hypertension (IIH) has only rarely been reported in this context. Objectives: To delineate the frequency and clinical and radiological features of, as well as the cause- and-effect relationship between, ACL-Ab and IIH. Methods: We analyzed the medical records of patients with IIH hospitalized between January 1989 and September 1995. All patients underwent magnetic resonance imaging or magnetic resonance venography or angiography. Excluded were patients with intracranial hypertension due to dural sinus thrombosis or traumatic, structural, neoplastic, or infectious disorders. Patients who were found on at least 2 separate occasions to have increased IgG titers of ACL-Ab were identified and compared with patients without ACL-Ab. Results: Six (43%) of 14 patients with IIH had ACLAb. No differences in clinical, laboratory, or radiological variables could be found between patients with and without ACL-Ab. Only 3 of the 11 ACL-Ab-positive patients had previous systemic or neurologic abnormalities associated with ACL-Ab. Conclusions: Anticardiolipin antibodies may cause IIH through mechanisms unrelated to major venous thrombosis. Idiopathic intracranial hypertension is frequently associated with ACL-Ab and can be the presenting symptom of the antiphospholipid syndrome. There are no major clinical, laboratory, or radiological features that distinguish between patients with IIH with and without ACL-Ab.