The Northwick Park Heart Study (NPHS) has demonstrated associations of high levels of factor VII coagulant activity (VIIc) and of plasma fibrinogen concentration with the risk of subsequent ischaemic heart disease (IHD). In cross-sectional data from the 2023 white men in NPHS, lifetime duration of smoking was a determinant of initial plasma fibrinogen levels. Fibrinogen levels had apparently begun to fall soon after smoking was discontinued but it was over 5 years before they had returned to levels found in life-long non-smokers. In prospective data, smoking cessation and the adoption or resumption of smoking were associated with a decrease or an increase, respectively, of about 0.15 g/l in plasma fibrinogen. These changes would lower or raise the risk of IHD by about 20%. A switch from cigarettes to cigars was associated with a large increase in fibrinogen. A substantial part of the relation between smoking and IHD appears to be mediated through the fibrinogen concentration. Following changes in body mass, VIIc rose in those who had given up smoking and fell in those who resumed.