A case of cerebral venous thrombosis caused by undiagnosed homocystinuria is reported. The pitfalls regarding the diagnosis of a potentially medically treatable condition are discussed. Cerebral venous thrombosis in children has a variable type of onset and a multiplicity of causes. This type of pathology, although not frequent, is more common than previously thought. Among the different etiologies, undiagnosed homocystinuria is not routinely considered. We report a case of venous thrombosis of the left transverse cerebral sinus in a girl with drug-resistant partial epilepsy and homocystinuria. This diagnosis was considered and confirmed after the appearance of acute cerebral symptoms caused by venous thrombosis.