The North Karelia Worksite Intervention Study was carried out to assess the effectiveness of worksite-based innovative intervention on chronic disease risk factors. The one-year intervention combined use of mass media, worksite opinion leaders, risk assessment and counselling and other health education measures. The study used eight medium sized intervention worksites (IW) and eight matched reference worksites (RW). Of the 715 workers 91% participated in the initial survey, and out of these 89% in the one-year follow-up survey. The proportion of current smokers changed from 39% to 30% (p less than 0.05) at the IWs while no change took place at the RWs (33%). The effect on smoking was confirmed by serum thiocyanate analyses. An additive score of the three main CHD risk factors changed from 3.1 to 2.7 at the IWs and from 3.2 to 3.0 at the RWs (p less than 0.05 for the net change). The results and experiences showed the feasibility of the intervention and significant, although modest, effects on risk factors and health behaviour, notably smoking.