Objectives: vascular lesions are important contributors to cerebral disease, yet relatively little work has been done on relationships between haematological factors and cognitive function. We have explored these relationships in the Caerphilly cohort of older men. Design: the development of vascular disease and the decline in cognitive function are being studied in a cohort of older men in Caerphilly, South Wales, UK. We have recorded a range of life‐style, dietary, lipid, haematological and rheological factors which are, or may be, predictive of vascular disease. We have examined the relationships between these and both incident myocardial infarction and stroke, and identified factors relevant to cognitive function. Setting: a community‐based study based upon a representative population sample of older men. We collected data for the study from around 90% of the survivors of the original cohort. Subjects: 2154 men who were aged 55–69 years at the time blood was taken for the haematological tests and cognitive function was tested. Main outcome measures: we present data from the AH4 test of general cognitive performance and from choice reaction time, a test of vigilance. Results: there is a U‐shaped relationship between haematocrit and cognitive function, the best cognitive function scores occurring at a mean haematocrit of 0.46. The relationship is significant for the choice reaction time (PPConclusions: blood rheology, as estimated by both haematocrit and plasma viscosity, is a significant determinant of cognitive function in older men. On the other hand, the thrombotic potential of blood, as indicated by fibrinogen level, shows no significant relationships. The relationships with rheology seem to be direct, presumably through blood flow at the time of testing, rather than through underlying long‐term disease processes.