A girl with Down's syndrome, moyamoya disease and sagittal sinus thrombosis is described. She was diagnosed after acute neurological deterioration by MRI and angiography. Recombinant tissue plasminogen activator (r-TPA) was injected locally to recanalise the thrombus. The patient's condition significantly improved and she was discharged. After 2 years of follow-up the child remains asymptomatic. Moyamoya syndrome and cerebral venous thrombosis should not be overlooked as a cause of acute neurological deterioration in a child with Down's syndrome. MRA appears to be a safe and accurate alternative to traditional angiography for the diagnosis of moyamoya disease. Local fibrinolysis with r-TPA is the treatment of choice for cerebral venous thrombosis due to its safety and efficacy.