The present study describes changes during the period 1982 to 1992 in smoking prevalence, knowledge of the health consequences of smoking and analysis factors predicting quitting smoking among Danish adults. Data were collected by questionnaire in two independent cross-sectional studies in the western part of the County of Copenhagen. In 1982 the participation rate was 79% among 4807 randomly selected men and women aged 30, 40, 50 and 60 years. In 1992 it was 73% among 2226 randomly selected men and women of similar ages. Five years later 2987 of the participants from the study in 1982 were re-examined. From 1982 to 1992 the proportion of participants stating that smoking increases the risk of bronchitis, asthma, lung cancer, cancer of mouth and throat, thrombosis and hypertension increased. Knowledge was independent of smoking status. In 1982 men and women with a vocational education were more knowledgeable than those who were uneducated. This difference equalized in men during the study period. During the same period, the prevalence of smoking decreased from 62% to 52% in men and from 54% to 49% in women, but the declining prevalence was found in those with a vocational education only and an existing educational difference in smoking behaviours was enhanced. The decline in smoking in Denmark in the last decade has been associated with a narrowed gender difference and widened social difference. Knowledge of the health consequences of smoking has increased independently of these changes in smoking behaviour.