Swimmer's migraine. Three cases of sudden, severe headache occurring during swimming are described. A 51-year-old female had been engaging in a swimming exercise for about 20 minutes when she suddenly experienced a pulsating headache in the parietotemporal region, accompanied by nausea. A few days later, she experienced a similar episode, again during swimming practice. A 45-year-old male developed a pulsating headache with nausea immediately after diving into a swimming pool, and had a similar attack during diving practice 1 week later. A 32-year-old male developed a pulsating headache accompanied by nausea while swimming in the sea. In all three cases, blood pressure, pulse rate, neurological findings, cervical spinal x-rays, brain CT scans, and hematological findings were normal and the outcome was good. Although these patients' headaches were diagnosed as benign exertional headache, pathophysiologically they appeared to resemble the headache associated with sexual activity. 