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To analyse the association between time of onset of subarachnoid haemorrhage and diurnal blood pressure variations of ambulant normo- and hypertensive subjects.Retrospective, population-based study.The population (246 000) of the Health Care District of Central Finland.During 1980-1987 a total of 332 subjects in the study population had their first subarachnoid haemorrhage. The hour of onset could be obtained for 287 patients, and these form the basis of the present study.The onset of subarachnoid haemorrhage occurred significantly more often during the waking hours than during the night. The correlation between the hourly numbers of patients suffering a haemorrhage and the corresponding mean systolic and diastolic blood pressure values of ambulant normo- and hypertensive subjects was highly significant (r=0.88, P<0.001). The results were similar when the 224 patients with proved aneurysmal bleed were analysed separately (r=0.79-0.85, P< 0.001).The diurnal blood pressure variations of ambulant normo- and hypertensive subjects, especially the transient blood pressure peaks reaching much higher levels of pressure during the waking hours than during the night, may be crucial in determining the time of rupture of a critically weakened aneurysm wall.