AIMS: To evaluate the effects of smoking and other lifestyle factors on body mass index (BMI), and changes in BMI in relation to changes in smoking status. METHODS: A cross-sectional study was performed on 10,920 males (3937 smokers) and 12,090 females (4343 smokers) who participated in the fourth Tromsø Study (performed in 1994-95). A longitudinal study was performed on 2364 males (732 smokers in 1994-95) and 2738 females (942 smokers in 1994-95) who participated in both the fourth and the fifth Tromsø studies (performed in 2001). RESULTS: In the cross-sectional study, current smokers of both genders had a lower BMI (25.0+/-3.4 vs. 25.5+/-3.2 kg/m(2) in males, and 23.9+/-3.9 vs. 25.3+/-4.6 kg/m( 2) in females, p<0.01), a lower degree of physical activity, and a higher consumption of coffee and alcohol than never-smokers. We found a U-shaped relationship between number of cigarettes smoked per day and BMI, with the lowest BMI in those smoking 6- 10 cigarettes per day. Heavy smokers and never-smokers had similar BMI. In the longitudinal study, continuing smokers had a smaller increase in BMI than those who gave up smoking. In those who gave up smoking, there was a significant, positive relationship between number of cigarettes smoked in 1994-95 and increase in BMI. CONCLUSIONS: There is a U-shaped relationship between number of cigarettes smoked per day and BMI. Smoking cessation is associated with an increase in weight as compared to those who continue smoking.