The objective was to study the association between smoking habits and mortality from coronary heart disease among men and women aged 35-49 years. Almost 45000 individuals (50% women) from three counties in Norway attended a screening programme during 1974-1978. The participation rate was almost 90% of the population. These individuals have been followed for an average of 18 years through the Norwegian death register. There were 1021 and 193 deaths from coronary heart disease among men and women, respectively. The mortality rate among current smokers was three times higher than that among those who had never smoked cigarettes. The relative risk between low-dose smokers and non-smokers was higher than that between high- and low-dose smokers. Among men, the relative risk between smokers and non-smokers was lower in the age group 45-49 years than in the two younger 5-year age groups. Among women, the relative risks did not vary significantly across the age groups. The relative risk between smokers and non-smokers remained the same in the first and second half of the observation period in men. A stronger association in the second half of the observation period was suggested among women (P = 0.08). The duration of smoking was an independent predictor of coronary heart disease among men, but not among women. Duration of smoking and length of follow-up seemed to have a different relationship with mortality from coronary heart disease in men and women. The dose-response relationship was the same in men and women, on the relative scale. Age affected the magnitude of relative risk between smokers and non-smokers among men, but not among women.