A 23-year-old man was admitted because of vomiting and severe, progressive headache. After admission, he suffered from a generalized clonic seizure, and developed right hemiparesis. Contrast-enhanced CT of the brain showed empty delta sign in the posterior part of the superior sagittal sinus and filling defect in the straight sinus. T2-weighted MRI demonstrated high intensity area in the left parieto-occipital subcortical region. Delayed venous phase of the right carotid angiography confirmed the diagnosis of thrombosis of the superior sagittal sinus. Coagulation studies gave a protein C activity of 35.3% (normal range 55-140%), protein C antigen of 45% (normal range 70-150%). Same results were obtained from the studies of his father and one of his sisters, indicating hereditary protein C deficiency. We started warfarin therapy under the administration of heparin for a week, then he has been followed up with no subsequent problems.