OBJECTIVE--To examine the relation between resting heart rate and new major ischaemic heart disease events in middle aged men with and without pre-existing ischaemic heart disease. DESIGN--Prospective study of a cohort of men with eight years follow up for cardiovascular morbidity and mortality for all men. SETTING--General practices in 24 British towns (the British Regional Heart study). SUBJECTS--7735 men aged 40-59 years drawn at random from the age-sex registers of one general practice in each town. MAIN OUTCOME MEASURES--Major ischaemic heart disease events such as sudden cardiac death, other deaths attributed to ischaemic heart disease, and non-fatal myocardial infarction. RESULTS--During the follow up period of eight years, 488 men had a major ischaemic heart disease event (217 fatal and 271 non-fatal). Of these, 117 were classified as sudden cardiac death (death within one hour of the start of symptoms). The relation between heart rate and risk of all major ischaemic heart disease events, ischaemic heart disease deaths, and sudden cardiac death was examined separately in men with and without pre-existing ischaemic heart disease. In men with no evidence of ischaemic heart disease, there was a strong positive association between resting heart rate and age adjusted rates of all major ischaemic heart disease events (fatal and non-fatal), ischaemic heart disease deaths, and sudden cardiac death. This association remained significant even after adjustment for age, systolic blood pressure, blood cholesterol, smoking, social class, heavy drinking, and physical activity, with particularly high risk in those with heart rate > or = 90 beats/min. The increased risk seen in those with increased heart rate was largely due to a significantly increased risk of sudden cardiac death, which was five times higher than in those with heart rate < 60 beats/min. The effect of heart rate on sudden cardiac death was present irrespective of blood pressure or smoking state. In men with pre-existing ischaemic heart disease a positive association was seen between raised heart rate and risk of all major ischaemic heart disease events, ischaemic heart disease death, and sudden cardiac death, but the effect was less noticeable than in men without pre-existing ischaemic heart disease. CONCLUSION--In this study of middle aged British men increased heart rate > or = 90 beats/min) is a risk factor for fatal ischaemic heart disease events but particularly for sudden cardiac death. The effect is not dependent on the presence of other established coronary risk factors and is most clearly seen in men free of pre-existing ischaemic heart disease at initial examination.