OBJECTIVE To investigate the relationship between physical activity, physical fitness, blood pressure, and fibrinogen. METHODS This was a cross sectional population study using a two stage probability sample. METHODS Northern Ireland. METHODS A sample of 1600 subjects aged 16-74 years from the population of Northern Ireland. METHODS Physical activity profile from computer assisted interview using the Allied Dunbar national fitness survey scales. Physical fitness using estimation of VO2 max by extrapolation from submaximal oxygen uptake while walking on a motor driven treadmill. Systolic and diastolic blood pressure measured with a Hawksley random zero sphygmomanometer. Measurement of fibrinogen using the Clauss method. RESULTS There were significant relationships between both current and past activity and blood pressure. These were of a magnitude that would have been clinically significant, but for the fact that, with the exception of the relationship between habitual activity and diastolic pressure (p = 0.03) and past activity and systolic pressure (p = 0.03) in men, they were not sustained after adjustment for the effect of age using analysis of variance. After adjustment for other potentially confounding factors using multiple regression, there was an inverse relationship between systolic blood pressure and past activity in men, so that those with a life-time of participation compared with a life-time of inactivity had a lower systolic blood pressure of 6 mmHg (p < 0.05). There was a highly significant (p < 0.001) inverse association between both systolic and diastolic blood pressure and physical fitness (VO2 max) which was not sustained after adjustment for possible confounding factors. There were relationships between fibrinogen and highest recorded activity (p < 0.001), habitual activity (p < 0.01), and past activity (p < 0.01) in men but no significant relationship in women. The relationship between fibrinogen and activity was no longer sustained after adjustment for possible confounding factors. There was a highly significant (p < 0.001) inverse relationship with physical fitness using VO2 max. This relationship was sustained after adjustment for possible confounding factors in both men (p < 0.05) and women (p < 0.001). CONCLUSIONS There was a relationship between physical activity, physical fitness, and blood pressure but the relationship was greatly influenced by age. A reduction of 6 mmHg in systolic blood pressure associated with past activity is of clinical significance and supports the hypothesis that physical activity is of benefit in reducing cardiovascular risk. There was a lower level of fibrinogen in those who were most active but this relationship was not significant after adjustment for possible confounding factors. There was also a lower level of fibrinogen those who were most fit (VO2 max) and this relationship persisted even after adjustment for possible confounding factors.