Aim: Carbon monoxide (CO) in blood as assessed by the COHb% is a marker of the cardiovascular (CV) risk in smokers. Non-smokers exposed to tobacco smoke similarly inhale and absorb CO. The objective in this population-based cohort study has been to describe inter-individual differences in COHb% in never smokers and to estimate the associated cardiovascular risk. Methods: Of the 8,333 men, aged 34—49 years, from the city of Malmö, Sweden, 4,111 were smokers, 1,229 ex-smokers, and 2,893 were never smokers. Incidence of CV disease was monitored over 19 years of follow up. Results: COHb% in never smokers ranged from 0.13% to 5.47%. Never smokers with COHb% in the top quartile (above 0.67%) had a significantly higher incidence of cardiac events and deaths; relative risk 3.7 (95% CI 2.0—7.0) and 2.2 (1.4—3.5), respectively, compared with those with COHb% in the lowest quartile (below 0.50%). This risk remained after adjustment for confounding factors. Conclusion: COHb% varied widely between never-smoking men in this urban population. Incidence of CV disease and death in non-smokers was related to COHb%. It is suggested that measurement of COHb% could be part of the risk assessment in non-smoking patients considered at risk of cardiac disease. In random samples from the general population COHb% could be used to assess the size of the population exposed to second-hand smoke.