ObjectivesThe aim of this study was to use the parametric g-formula to estimate the 19-year risk of myocardial infarction (MI) under hypothetical interventions on six cardiovascular risk factors.Design and settingA populations-based cohort study with repeated measurements, the Tromsø Study.Primary outcome measureMyocardial infarction.ParticipantsWe estimated the relative and absolute risk reduction under feasible and intensive risk reduction strategies for smoking, physical activity, alcohol drinking, body mass index, total serum cholesterol and systolic blood pressure in 14 965 men and women with 19 years of follow-up (1994–2013).ResultsThe estimated 19-year risk of MI under no intervention was 7.5% in individuals with baseline mean age 49.3 years (range 25–69). This risk was reduced by 30% (95% CI 19% to 39%) under joint feasible interventions on all risk factors, and 70% (60%, 78%) under a set of more intensive interventions. The most effective interventions were lowering of total cholesterol to 5.18 mmol/L and lowering of systolic blood pressure to 120 mm Hg (33% and 37% lower MI risk, respectively). The absolute risk reductions were significantly larger in men, in older participants, in smokers and in those with low education.ConclusionModification of population levels of cardiovascular risk factors could have prevented close to one-third of the cases of MI in the municipality of Tromsø during 19 years of follow-up.