The role of hypertension in the increased rate of heart attack reported in vigorously active subjects was examined in a large prospective study of 7735 middle-aged men drawn from general practices in 24 British towns (The British Regional Heart Study). Analyses were restricted to 5694 men with no evidence of pre-existing ischaemic heart disease or stroke at screening, in whom there were 311 major ischaemic heart disease events after 9.5 years follow-up. Risk of major ischaemic heart disease events decreased significantly with increasing physical activity to levels of moderate/moderately vigorous activity, with a 50% reduction in risk compared with inactive men after adjustment for age, body mass index, smoking, heavy drinking, social class and blood cholesterol. However, at the highest level of physical activity (vigorous group) risk of major ischaemic heart disease events was increased above that seen in the moderate/moderately vigorous group (rr = 1.68, P = 0.05). When separated into normotensives (n = 3888) and hypertensives (n = 1806; SBP > or = 160 mmHg or DBP > or = 90 mmHg or on regular antihypertensive treatment), the increased risk of major ischaemic heart disease events in the vigorous group was only evident in hypertensive men. They showed more than a twofold increase in risk compared with the moderate and moderately vigorous group (rr = 2.7, P < 0.05). In normotensive men, risk was significantly lowered in those engaged in moderate activity with no further decline in rate of heart attack at increasing levels of physical activity.(ABSTRACT TRUNCATED AT 250 WORDS)