In the Danish Aneurysm Study 1,076 patients were admitted with an aneurysmal subarachnoid haemorrhage in the 5-year period 1978–83. Pre-existing arterial hypertension, defined by the necessity for antihypertensive treatment at the time of admission was identified in 155 patients (15%). Nine hundred and two (85%) were normotensive. Nineteen patients were excluded because of inadequate information. Comparisons between the hypertensive and normotensive groups of patients showed no significant differences in sex-distribution, onset and course of the initial insult, results of primary CT-scan, location, size and numbers of aneurysms, number of rebleedings, and also in neurological symptoms, mental status, accommodation and occupation at a 2-year follow-up examination. The hypertensive group differed significantly in the following variables from the normotensive group. The median age was 8 years older (55 and 47 years of age respectively); a fewer number of patients fell into Hunt grade I-II on admission (34% and 43% respectively); an increased frequency of extracranial (23% and 13% respectively) and intracranial (35% and 18%, respectively) atherosclerosis was seen on angiography; a fewer number of patients underwent operation (48% and 66%, respectively); and at the 2-year follow-up examination an increased rate of overall mortality was evident (59% and 42%, respectively). There was an increased rate of mortality in patients in Hunt grade I-II on admission (52% and 22%, respectively) and an increased rate of mortality in patients who rebled (100% and 75%, respectively). Pre-existing arterial hypertension is an unfavourable prognostic factor after an aneurysmal subarachnoid haemorrhage.