Cohort and case control studies show a 30% excess risk of ischemic heart disease in nonsmokers whose spouses smoke compared with that in nonsmokers whose spouses do not smoke. There is a nonlinear dose-response; the excess risk from actively smoking 20 cigarettes/day is only 80%. Large cohort studies of active smoking support the nonliner dose-response (the excess risk in smokers of 5 cigarettes/day is about 50%). Animal studies show a pronounced vascular effect of environmental tobacco smoke. In experimental studies passive and active smoking have similar effects on platelet aggregation. The collective evidence supports a significant effect of low dose tobacco smoke exposure in causing ischaemic heart disease.