**OBJECTIVE: **To estimate trajectories of body mass index (BMI) and determine their association with incident frailty in later life.

**METHODS: **Data come from the 2004 to 2012 waves of the Health and Retirement Study, a longitudinal survey of older adults. Analysis was restricted to respondents who were not frail at baseline (n = 10,827). BMI (kg/m(2) ) was calculated from self-reported weight and height. Incident frailty was assessed using the Frailty Index. Longitudinal growth mixture modeling was used to estimate the relationship between BMI trajectories and incident frailty over a 10-year period.

**RESULTS: **Four trajectory classes were identified: weight gain (n = 162 [1.4%], mean final BMI = 42 kg/m(2) ), weight loss (n = 171 [1.7%], mean final BMI = 25.0 kg/m(2) ), consistent obesity (n = 640 [6.8%], mean final BMI = 34.7 kg/m(2) ), and consistent overweight (n = 9,864 [90.1%] mean final BMI = 26.0 kg/m(2) ). Cumulative incidence of frailty was 19.9%. Relative to the consistent overweight class, the weight gain class had the highest likelihood of incident frailty (odds ratio, OR: 3.61, 95% confidence interval, CI: 2.39-5.46). The consistent obesity (OR: 2.72, 95% CI: 2.06-3.58) and weight loss (OR: 2.81, 95% CI: 1.84-4.30) classes had similarly elevated risk of frailty.

**CONCLUSIONS: **Weight change and obesity are associated with risk of frailty.