Changes in smoking habits in Denmark from 1953 to 1991 are described on the basis of data from three nationwide population surveys on health and morbidity. The data on smoking habits were collected at personal interviews with 34,018, 4753 and 4818 subjects aged more than 15 years in 1953-1954, 1986-1987 and 1990-1991, respectively. Among men, the proportion of smokers has fallen from 78% to 47%, but among women it has remained constant at around 40%. The greatest decrease occurred among younger men. The proportion of heavy smokers (> or = 15 g tobacco a day) has remained constant at around 25% in men, but in women it has increased from 3% to 14%. There is a correlation between occupational status and smoking; the proportion of smokers being highest among the unemployed, early retirement pensioners and unskilled workers, and lowest among upper-level salaried employees. Today, more smokers started smoking before the age of 15 years. Possible explanations of the changes include public debate, campaigns concerning the health risks of tobacco, and a change in the roles of the sexes. Since today's smokers are characterized by smoking onset before age 15 years it is proposed that future initiatives to reduce smoking must focus on the psychosocial factors related to early onset of smoking.