Carotid angiograms in 187 patients presenting with transient ischaemic attacks and minor completed strokes were reviewed. A haematocrit of 50% or more was encountered more frequently in those found to have carotid occlusion. The severity of vessel-wall disease showed no relationship to the level of haematocrit. Computerised axial tomographic measurements of the volume of the cerebral infarct seen in 23 patients with carotid occlusion and completed strokes showed a correlation between the size of the infarct and the height of the haematocrit. It is suggested that reduced blood flow resulting from increased blood viscosity associated with a high haematocrit adversely affects collateral flow, thereby increasing the size of the infarct.