A 63-year-old man was admitted to our hospital because of convulsive seizures. Radiological examinations revealed cerebral venous sinus thrombosis in the anterior part of the superior sagittal sinus. He had marked hyperhomocysteinemia (93.5 nmol/ml) due to combined deficiencies of folate and vitamin B12. He was T/T homozygous for methylene tetrahydrofolate reductase C677T polymorphism. He received a supplement therapy of vitamins. First, he was administered folate orally. After 3 months, the serum level of homocysteine decreased to 22.6 nmol/ml (an 86% reduction), but was still above the normal level. Next, an additional supplement therapy of vitamin B12 lowered the homocysteine level to normal (12.3 nmol/ml) after 4 months. These results showed that the increase of homocysteine levels in this patient was mainly caused by the deficiency of folate. Additionally, acquired risk factors like vitamin deficiencies increased the level of serum homocysteine to almost 100 nmol/ml.