A 42-yr-old woman with hypertension and renal involvement due to systemic lupus erythematosus (SLE) developed unilateral headache followed by the sudden onset of confusion and a grand mal convulsion. Cerebral computed tomography was normal. A magnetic resonance imaging angiogram revealed cerebral venous thrombosis and a venous infarct. Nephrotic syndrome had resulted in an acquired protein S deficiency. A review of previous cases suggests that either renal disease with proteinuria or features of the antiphospholipid syndrome are prerequisites for the development of cerebral venous thrombosis in SLE. Low free-protein S levels may be an additional risk factor. Furthermore it is likely that this condition is underdiagnosed.