SUMMARY In a hospital-based series of 66 consecutive patients with non-progressive cerebral infarction, the time of onset and the type of infarction on computed tomography were studied retrospectively. Forty-six (78%) patients suffered cerebral infarction between 6am and 6pm. Only five patients (8%) had their infarct between midnight and 6am. Only three patients had a watershed-infarct, and these occurred during the daytime. Our results do not support the belief that atherothrombotic brain infarction is largely determined by haemodynamic factors.Traditional teaching holds that cerebral infarction occurs more often at night than during the daytime and that symptoms are usually noticed on waking. This, in combination with the fact that blood pressure is lower during the night, fits the notion of cerebral "thrombosis". However, the evidence that cerebral infarction is a nocturnal illness1'2 is not convincing.In those studies, the diagnosis of cerebral infarction was imprecisely defined, or more importantly, patients with incomplete information have not been taken into account. To shed some new light upon this problem, we have, in a retrospective review of patients recently admitted with cerebral infarction, attempted to retrace the exact time of onset in each patient. In addition, we studied the type of infarction (in the territory of a single artery or between different territories), as this provides some evidence for embolism or haemodynamic factors, respectively. Patients and methodsBetween 1 January 1984 and 1 September 1985, 99 patients were diagnosed as having cerebral infarction. To facilitate comparison with one previous study,' we adopted the same criteria for "stroke of acute onset", in that symptoms should have reached their maximum in less than 6 hours and should Address for reprint requests: Prof J van Gijn,