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Homocystinuria, an autosomal recessive defect in methionine metabolism, is responsible for a multi-systemic disorder. If not treated with either pyridoxine or betaine, patients have a high risk of arterial or venous thrombotic events before the age of 30.A 16 year-old woman with pyridoxine-non-responsive homocystinuria presented with a first seizure and headaches. She had bilateral papilloedema, no other deficits. MR venogram was normal, CSF opening pressure was 50 cmH20. She was treated as Idiopathic Intracranial Hypertension. Two months later she represented with behavioural changes. A new venogram showed extensive venous sinus thrombosis and right thalamic haemorrhagic infarction. Her homocysteine levels were high. She was anticoagulated. Still complaining of headaches, an MRI at one year showed new extensive diffuse white matter changes. She had been on regular betaine treatment, but, non-compliant with a low-protein diet, her methionine levels were high (>600 micromol/L). Extensive white matter changes have been reported in association with high levels of methionine, and this is the most likely diagnosis in our patient.Only a few cases of methionine-induced cerebral white matter changes are reported in literature. Regression of the lesions and improvement of symptoms can occur after strict adherence to a low-protein diet.