Objectives: To examine in an older population all-cause and cause-specific mortality associated with underweight (body mass index (BMI)<18.5), normal weight (BMI 18.5-24.9), overweight (BMI 25.0-29.9), and obesity (BMI≥30.0). Design: Cohort study. Setting: The Health in Men Study and the Australian Longitudinal Study of Women's Health. Participants: Adults aged 70 to 75, 4,677 men and 4,563 women recruited in 1996 and followed for up to 10 years. MEASUReMENTS: Relative risk of all-cause mortality and cause-specific (cardiovascular disease, cancer, and chronic respiratory disease) mortality. Results: Mortality risk was lowest for overweight participants. The risk of death for overweight participants was 13% less than for normal-weight participants (hazard ratio (HR)=0.87, 95% CI=0.78-0.94). The risk of death was similar for obese and normal-weight participants (HR=0.98, 95% CI=0.85-1.11). Being sedentary doubled the mortality risk for women across all levels of BMI (HR=2.08, 95% CI=1.79-2.41) but resulted in only a 28% greater risk for men (HR=1.28 (95% CI=1.14-1.44). Conclusion: These results lend further credence to claims that the BMI thresholds for overweight and obese are overly restrictive for older people. Overweight older people are not at greater mortality risk than those who are normal weight. Being sedentary was associated with a greater risk of mortality in women than in men.