A circadian pattern for the onset of myocardial and cerebral infarction has been identified. To evaluate this phenomenon further, we analyzed prospectively collected data from 151 patients with acute ischemic stroke. The number of strokes per 6-hour period were the following: midnight to 6 AM, 20 (13%); 6 AM to noon, 86 (57%); noon to 6 PM, 21 (14%); and 6 PM to midnight, 24 (16%). This pattern was not affected by previous use of aspirin. The most frequent time of onset was 6 AM to noon for all subgroups of ischemic stroke: small artery, 71%; cardioembolic, 62%; large artery atherothrombotic, 57%; large artery atheroembolic, 46%; and "other" or unknown cause, 40%. We also investigated the time between awakening and stroke onset in 145 patients and found that 24% of ischemic strokes occurred within the first hour after awakening. Our data demonstrate that an early morning peak exists for all subtypes of stroke. Our data also suggest that the most critical period is the first hour after awakening.