The significance of antiphospholipid antibodies was examined in patients with childhood strokes. Eight patients, aged 2-13 years, who presented with acute hemiplegia were studied. On the basis of magnetic resonance imaging, magnetic resonance angiography, single photon emission computed tomography, and cerebral angiographic findings, 3 children were diagnosed as having infarctions due to moyamoya disease, and the others as having idiopathic infarctions. Serologic studies revealed elevated anticardiolipin antibody (ACA) IgG in 3 of the 5 patients with idiopathic infarction; no such elevation was revealed in patients with moyamoya disease. Values for all other tests, including ACA IgM and lupus anticoagulant, were within normal limits or negative in all patients. ACA IgG, therefore, may be a more important causative agent of childhood strokes than was previously considered.