Thunderclap headache is an acute high intensity headache similar to that seen in the setting of ruptured saccular aneurysm. We report four patients without subarachnoid hemorrhage who presented with thunderclap headache. Three patients had transient neurologic signs or symptoms. Cerebral angiography revealed diffuse segmental intracerebral arterial vasoconstriction which was reversible in the one patient in whom angiography was repeated. The headaches resolved spontaneously in all cases and after 1 week did not recur. These cases highlight the specific clinical and angiographic features and the self-limited course of patients with thunderclap headache and suggest that thunderclap headache may represent a unique headache category. On the basis of these cases and others reported in the literature, we propose diagnostic criteria for thunderclap headache.