Hazard ratios (all-cause mortality) with 95% confidence interval for interactions with BMI category. Interaction with the type of BMI assessment (A), age group (B), smoking status (C) and sex (D) and adjustment for age at baseline, age2, sex, study waves, smoking status. Reference categories are participants with BMI between 20 and <22.5 kg/m2 and either measured BMI (A), aged between 25 and 45 years (B), non-smokers (never and former) (C) or women (D). 31,528 participants (25–74 years at baseline) of the Swiss MONICA, NRP 1A, SOMIPOPS and SHS 92/93. Number of deaths in the underweight group: A) measured BMI: 56, self-reported BMI: 72; B) aged between 25 and 45 years: 28, aged between 46 and 74 years: 100; C) non-smokers: 70, current smokers: 58; D) men: 24, women: 104.