A 23-year-old woman with known tension-type headache and in treatment with oral contraceptives was hospitalized with headache, nausea and vomiting during two weeks. Prior to hospitalization the symptoms had been interpreted as migraine. Blood samples including D-dimer were normal. A computed tomography showed cerebral venous thrombosis (CVT), so the patient was treated with anticoagulation drugs and her symptoms remitted slowly. CVT is a dangerous condition, and the symptoms can have a fluctuating course which can mask the diagnosis. D-dimer cannot be used to exclude CVT, and attention should be paid to the risk factors.