The diurnal variation in the onset of stroke was examined in 557 consecutive patients aged over 70 years. These included 194 patients with subarachnoid hemorrhage, 118 with intracerebral hemorrhage, and 245 with thromboembolic cerebral infarction. All three types of strokes exhibited a peak incidence between 1000 and 1200 hours. Intracerebral hemorrhages occurred less frequently between 0400 and 0600 hours, but there were no differences between the groups for the other time periods. There was no difference in the time of onset of stroke between normotensive and treated or untreated hypertensive patients. There were more untreated hypertensive patients in the intracerebral hemorrhage group than in the other stroke type groups. Subarachnoid hemorrhage occurred more frequently in the lavatory and during sexual and sporting activity. Intracerebral hemorrhage occurred more commonly during driving or the consumption of alcohol. Cerebral infarction occurred more frequently during sleep or was noticed on waking. No differences were found for the other activities examined. The relationship between diurnal variation in stroke and the known variation in blood pressure is discussed.