BACKGROUND. The aims of the present study were to provide lifetime smoking data for epidemiological studies of tobacco-induced cancer in Norway, and to sort out the influence of age, gender, and period on the population's smoking habits in this century. METHODS. We used annual surveys of smoking habits from 1954 to 1992, and individual lifetime smoking histories collected in 1965 from a population sample born 1893-1927. The population was divided into 5-year sex-and-birth cohorts born between 1890 and 1974, and smoking habits were described as the proportion of current smokers in 5-year age groups between 15 and 74 years old. We also estimated the average tobacco consumption per smoker by calendar year after 1930. RESULTS. The proportion of smokers increased with each succeeding cohort of men and women until the 1950s, when the highest proportion of male smokers (76-78%) was observed among those born 1915-1934. This peak was followed by a decline in both men and women from 1955 to 1965. A second peak occurred in women around 1970, during which the highest proportion of smokers (52%) was observed in women born 1940-1949. From 1970 to 1990 smoking has declined in all cohorts of men and women, but at a slower rate after 1980 in the younger cohorts. The smoking proportion was more than five times higher in men than in women born 1890-1894, but the gap has declined with each succeeding cohort until no gender difference was present among those born after 1950. The average tobacco consumption per smoker between 1930 and 1950 remained fairly constant around 8.5 g per day in male smokers and 6 g per day in female smokers, followed by a continuous increase to 15 g per day in men and 12 g in women in 1985. CONCLUSIONS. The smoking habits in Norway appear to have been strongly influenced by social changes and the increasing awareness of the health hazards of smoking. Each cohort's response to these events has depended on the members' age and sex at the time.