Background. Reversible cerebral vasoconstriction syndrome is characterized by thunderclap headache associated with multifocal vasoconstriction of cerebral arteries in patients without aneurysmal subarachnoid hemorrhage (SAH). The vasoconstriction reverts within three months. Case report. A 44-year-old man had a thunderclap headache during sexual intercourse. A similar new episode occurred at rest 36 hours later. The patient had already experienced a thunderclap headache 10 years earlier, during coitus. There were no abnormalities on examination. His brain computed tomography scan was normal and cerebrospinal fluid analysis showed no xanthochromia, 15 WBC/mm3 and 10 RBC/mm3. Lumbar puncture was repeated two days later (WBC = 3/mm3 and RBC = 43/mm3). An initial digital cerebral angiography showed a diffuse segmental intracerebral vasospasm. When repeated, it was normal. He remains headache-free after twenty months. Conclusion. Reversible cerebral vasoconstriction syndrome is a secondary cause of thunderclap headache. Patients who have thunderclap headache with normal brain CT and cerebrospinal fluid without xanthochromia should be investigated for this syndrome.