Background Blood pressure is positively associated with ischaemic heart disease (IHD) mortality, but the strength of the association declines with age, and may differ between lean and obese people. Objective To study the association of blood pressure with IHD mortality stratified by attained age (<65 and ≥65 years) and by body mass index (BMI). Design Prospective cohort study. Setting General population with baseline measurements in 1984–1986. Participants 34 633 men and 36 749 women. Measurements Standardised measurements of blood pressure and BMI conducted by trained personnel, and information on potentially confounding factors was retrieved from self-administered questionnaires. Information on deaths from IHD was obtained from the Causes of Death Registry in Norway from baseline until the end of 2004. Results During 21 years of follow-up, 2529 men and 1719 women had died from IHD. The association of blood pressure with IHD mortality was stronger in people younger than 65 years than in older age groups (p for interaction, 0.001), and the association was further modified by BMI (p for interaction, 0.001). In this age group, the RR of death from IHD associated with systolic pressure ≥160 mm Hg in lean (BMI<25) people was 5.8 (95% CI 3.8 to 8.7) compared with the reference (systolic pressure 120–139 mm Hg and BMI <25), and in overweight (BMI 25–29) and obese (BMI≥30) people, the corresponding relative risks were 2.4 (95% CI 1.6 to 3.5) and 1.6 (95% CI 0.9 to 2.8), respectively. Conclusion The association of blood pressure with IHD mortality is modified by age and body mass index.