Anticardiolipin antibody (aCL) has been associated with thromboembolic phenomena, including stroke, in certain patients with systemic lupus erythematosus (SLE); however, the relation between this antibody and the central nervous system manifestations of SLE is unknown. Serum samples and cerebrospinal fluid from five patients with SLE and acute central nervous system manifestations were assayed for the presence of aCL. Anticardiolipin antibody was identified in sera from four of the five patients but in none of the cerebrospinal fluid samples. Nuclear magnetic resonance imaging showed 'infarct-like' lesions in these four patients. This preliminary study suggests that a correlation between serum aCL and cerebral infarcts in central nervous system lupus may potentially exist. From this limited study it seems unlikely that aCL has a direct pathogenic role in the diffuse encephalopathy of acute central nervous system lupus.