The use of relative and absolute effect estimates has important implications for the interpretation of study findings. Likewise, examining additive and multiplicative interaction can lead to differing conclusions about the joint effects of two exposure variables. The aim of this paper is to examine the relationship between BMI and mortality on the relative and absolute scales and investigate interaction between BMI and age. Data from 68,132 participants in the Women’s Health Initiative (WHI) study were used. The risk ratio and risk difference of BMI on mortality were estimated. A product term was also included to examine interaction between BMI and age on the multiplicative scale, and the relative excess risk of interaction was calculated to measure additive interaction. Results demonstrated that the mortality risk ratio decreased as women aged, but the mortality risk difference increased as women aged. Evidence of additive and multiplicative interaction between age and BMI was found. In postmenopausal women, the relative mortality risk associated with high BMI decreased with increasing age, but the absolute risk of high BMI increased with increasing age. This indicates the importance of considering the interaction between age and BMI to understand mortality risk in older women.