We experienced 3 cases of characteristic headache induced by swimming. The first case was a 51-year-old woman who suffered from a sudden attack of throbbing headache in the parieto-temporal region, accompanied by nausea, after 20 minutes of swimming practice in a pool. The headache disappeared after about 3 hours of bed rest. However, soon after she began to practice swimming in the pool a few days later, throbbing headache in the same region recurred. Though the headache improved upon bed rest, dull pain was persistent till bed time that evening. Two days later, as she squatted down with alacrity at her home, she felt intense pain in the whole head as if struck with a hammer. She was seen at our neurological clinic the following day but was found to be neurologically normal with a blood pressure of 130/70 and a pulse rate of 78/min. Blood chemistry tests, hematology, head digital subtraction angiography, cerebral CT and X-ray films of the cervical spine were all normal. After swimming was prohibited, the prognosis was good and the patient experienced no episode of headache at all during the intervening period of 2 years and 5 months after the first visit. The second case was a 45-year-old man who felt throbbing headache affecting the whole head immediately after diving in a swimming pool. The headache improved after about 3 hours of bed rest. One week later, an attack of similar headache occurred immediately after a dive in the swimming pool.(ABSTRACT TRUNCATED AT 250 WORDS)