Reversible posterior leucoencephalopathy and cerebral venous thrombosis share many symptoms. Both of them may lead to coma, and cause epilepsy or focal neurological signs. Moreover, diffuse leucoencephalopathy can be observed in both cases. Cerebral venous thrombosis needs anticoagulation which is not a riskless treatment. We describe a case of reversible posterior leucoencephalopathy in an hypertensed, seventy-year old man, presenting with a left lateral sinus hypoplasia whose clinical history and paramedical results first suggested a cerebral veinous thrombosis. Our case shows the misleadings a congenital vascular asymmetry can induce when confronted with a subacute coma.