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Objectives To examine how body mass index ( BMI) and change in BMI are associated with mortality in old (70-79) and very old (≥80) individuals. Design Pooled data from three multidisciplinary prospective population-based studies: OCTO-twin, Gender, and NONA. Setting Sweden. Participants Eight hundred eighty-two individuals aged 70 to 95. Measurements BMI was calculated from measured height and weight as kg/m2. Information about survival status and time of death was obtained from the Swedish Civil Registration System. Results Mortality hazard was 20% lower for the overweight group than the normal-underweight group (relative risk ( RR) = 0.80, P = .011), and the mortality hazard for the obese group did not differ significantly from that of the normal-underweight group ( RR = 0.93, P = .603), independent of age, education, and multimorbidity. Furthermore, mortality hazard was 65% higher for the BMI loss group than for the BMI stable group ( RR = 1.65, P < .001) and 53% higher for the BMI gain group than for the BMI stable group ( RR = 1.53, P = .001). Age moderated the BMI change differences. That is, the higher mortality risks associated with BMI loss and gain were less severe in very old age. Conclusion Old persons who were overweight had a lower mortality risk than old persons who were of normal weight, even after controlling for weight change and multimorbidity. Persons who increased or decreased in BMI had a greater mortality risk than those who had a stable BMI, particularly those aged 70 to 79. This study lends further support to the belief that the World Health Organization guidelines for BMI are overly restrictive in old age. [ABSTRACT FROM AUTHOR]