1. In a survey of blood pressures taken over 24-hr periods by automatic monitoring in a series of men with ischaemic cerebrovascular disease, the variances of the readings were used to compare the lability of blood pressure in subjects subdivided according to the territory of the vascular disorder.
2. The findings that diastolic variance during the daytime in vertebrobasilar ischaemia was significantly higher than in controls and that in both carotid and vertebrobasilar ischaemia, but not controls, daytime variance was higher than night variance, were explicable on the basis that variance is directly related to the height of the blood pressure.
3. There was no evidence that cerebral ischaemia itself, whether in the carotid or vertebrobasilar territories, was associated with abnormal blood pressure lability.
4. It did not appear likely that there was any significant difference between blood pressure lability in subjects who had had cerebral infarction and those who had had transient ischaemic attacks.