Objectives: To assess the relationship between haematocrit and risk of major ischaemic heart disease events.
Design: Prospective study of a cohort of men followed up for 9.5 years.
Setting: General practices in 24 towns in England, Wales, and Scotland (British Regional Heart Study).
Subjects: Altogether 7735 men aged 40-59 years at screening, who were selected at random from one general practice in each of 24 towns, were studied.
Main outcome measures: Fatal and nonfatal ischaemic heart disease events.
Results: Risk of major ischaemic heart disease events was significantly increased at haematocrit levels of > or = 46.0%. Men with raised haematocrit (> or = 46.0%) showed a 30% increase in relative risk (RR) of major ischaemic heart disease events (RR = 1.32; 95% confidence intervals (CI) 1.10,1.57, p < 0.01) compared with those with values below 46.0%, even after adjustment for age, social class, smoking, body mass index, physical activity, blood cholesterol, lung function (FEV1), and pre-existing evidence of ischaemic heart disease. Further adjustment for systolic blood pressure reduced the risk slightly (RR = 1.27; 95% CI 1.06,1.51, p = 0.02) but it remained significant. The relationship was seen in men with and without pre-existing evidence of ischaemic heart disease. The study suggests that an increased haematocrit level plays a part in the development of major ischaemic heart disease events.