Objective: To examine the relationship between antiphospholipid antibody positivity (expressed as the lupus anticoagulant) and cognitive dysfunction in patients with systemic lupus erythematosus (SLE). Methods: Cross-sectional comparisons of lupus anticoagulant (LA) positive (N = 39) and negative (N = 79) patients and controls (N = 35) on a cognitive test battery; 22 LA-positive and 53 LA-negative patients who had never experienced neuropsychiatric events (never-NP–SLE) were also compared separately. Results: LA-positive patients were 2 to 3 times more likely than were LA-negative patients to be designated as cognitively impaired. As a group, LA-positive patients, particularly those in the never-NP–SLE group, demonstrated lower performance primarily on tasks of verbal memory, cognitive flexibility, and psychomotor speed. Conclusions: LA positivity is associated with subclinical nervous system compromise, and a pattern of deficits compatible with subcortical involvement, possibly on the basis of ongoing LA-related microthrombotic events or vasculopathy. (JINS, 1997, 3, 377–386.)