During the autumn 1989, a material of 1,330 men and 1,561 women aged 20-65 years and resident in the County of Vejle replied to a questionnaire about their smoking habits and knowledge in connection with the prophylaxis of cardiovascular disease. 47% of the men and 39% of the women stated that they smoked. During the past year, 44% of the male and 51% of the female smokers and ex-smokers had altered or considered altering their smoking habits. The main motives for altering smoking habits were health or financial reasons. The main reasons that an attempt to alter smoking habits did not succeed were side effects (restlessness/nervousness), the force of habit or that the desire was too great. The attitude that smoking was of significance for cardiovascular disease was related to non-smokers. In addition, a connection was observed between the knowledge that smoking is a risk factor for cardiovascular disease that the fact that these participants did not smoke. No interactions were observed between knowledge, attitudes and smoking habits i.e. that the attitude that it was important to stop smoking was identical among smokers and non-smokers, regardless of whether smoking was deleterious to health. On the basis of previous attempts to alter smoking habits, the conclusion was drawn that this is a difficult task. One of the reasons is that the cause of smoking are partially unknown and that smoking is probably an important factor in various social situations. In addition, the motivation to alter smoking habits is more than dominated by the discomfort involved.(ABSTRACT TRUNCATED AT 250 WORDS)