Higher body mass and obesity are associated with bodily pain, and rates of chronic pain increase among older adults. Most past studies are cross-sectional, precluding determination of the temporal relationship between body mass and pain. A longitudinal study of body mass and pain among middle-aged adults found that higher body mass index (BMI) led to greater lower back pain. No longitudinal study of BMI and pain has been conducted among adults older than 70 years. This study used dual change score models to determine the directional relationship between BMI and bodily pain in a sample of middle-aged and older adults. Participants (n = 1889) from the Swedish Twin Registry (baseline age range 40-93 years) completed at least 1 nurse assessment of BMI and self-report ratings of pain interference and joint pain. Pain interference was not associated with BMI, but joint pain was analyzed in univariate and bivariate models, with dual change score models modeling the relationship of BMI and joint pain across age, both independently and as part of bivariate relationships. The results indicated a reciprocal relationship between BMI and joint pain, but joint pain generally led to changes in BMI. In addition, the relationship changed with age, until approximately age 80 years, increasing joint pain contributed to higher BMI, but after that time increasing joint pain contributed to lower BMI. In addition, sex differences in the relationship between BMI and pain appeared after age 70 years. Thus, joint pain contributes to changes in BMI among middle-aged and older adults, but the relationship may change by age and sex.