BACKGROUND: Cross-sectional analyses of national data have found that persons with high baseline body mass index (BMI) gain weight faster than persons at the median and that those whose weight was below the median gain very little weight. However, it is not clear whether these population-level changes reflect patterns at the individual level. METHODS: We examined longitudinal changes in BMI in initially underweight, normal-weight, overweight, and obese US men and women using individual-level repeat data from the Health and Retirement Study (n = 15,895; age range, 40-69 years at baseline). Linear mixed-effect regression was used to model 6-year change in self-reported BMI during 4 study periods (1992/1994-1998/2000, 1996/1998-2002/2004, 2000/2002-2006/2008, and 2004-2010). RESULTS: In the first 6-year period, the mean increase in BMI was greatest among persons who were initially normal weight (0.3 kg/m [95% confidence interval = 0.2 to 0.4]) and overweight (0.2 kg/m [0.1 to 0.3]). Weight gain accelerated in these groups with each subsequent period. Weight gain was less for initially class-I obese participants, and a net decrease in BMI was observed for class-III obese participants. CONCLUSION: These analyses suggest that the change in mean BMI among middle-aged and older US adults between 1992 and 2010 resulted mainly from accelerated weight gain among persons who were initially normal weight and overweight.