Overweight-obesity and smoking are two main preventable causes of premature death. Because the relationship between smoking and body mass index (BMI) complicates the interpretation of associations between BMI and death risks, direct estimates of risks associated with joint exposures are helpful. We have studied the relationships of BMI and smoking to middle age (40-69 years) death risk-overall and by causes-in a Norwegian cohort of 32,727 women and 33,475 men who were 35-49 years old when baseline measurements and lifestyle information were collected in 1974-1988. Individuals with a history of cancer, cardiovascular disease or diabetes at baseline were excluded. Mortality follow-up was through 2009. The relationship between BMI and middle age death risk was U-shaped. Overall middle age death risks were 11% in women and 21 % in men. The combination of obesity and heavy smoking resulted in fivefold increase in middle age death risks in both women and men: For women middle age death risk ranged from 6 % among never smokers in the 22.5-24.9 BMI group to 31% (adjusted 28%) in obese (BMI > 30 kg/m(2)) heavy smokers (≥20 cigarettes/day). The corresponding figures in men were 10% and 53% (adjusted 45%). Obese never smokers and light (1-9 cigarettes/day) smokers in the 22.5-24.9 BMI groups both experienced a twofold increase in middle age risks of death. For women, cancer (56%) was the most common cause of death followed by cardiovascular disease (22%). In men, cardiovascular disease was most common (41%) followed by cancer (34%). Cardiovascular disease deaths were more strongly related to BMI than were cancer deaths.