Objective US birth and longer length of US residence among the foreign-born have been linked to higher anthropometric measures. However, previous studies have been cross sectional and few have examined heterogeneity by ethnic group. Cross-sectional findings that show immigrant weight converging to US-born levels with longer time in the United States imply that immigrants' weight is increasing at a faster rate relative to US-born individuals. Prospective studies are necessary to confirm this pattern. Design and Methods Using longitudinal data from 1,486 Hispanic and 802 Chinese adults aged 45-84 years in the Multi-ethnic Study of Atherosclerosis, we examined whether foreign-born participants experienced greater increases in BMI and waist circumference (WC) than the US-born over a median follow-up of 5 years. We also investigated heterogeneity in these associations by Hispanic subgroup. Results Among Hispanics and Chinese, the foreign-born had a lower adjusted mean BMI and WC at baseline than the US-born, but there were no significant differences in BMI or WC change over time. There was heterogeneity by Hispanic subgroup: despite small baseline nativity differences in WC, foreign-born Mexican Hispanics had a greater annual mean increase in WC over time compared to US-born Mexican Hispanics (mean difference in annual change = 0.28 cm, P = 0.03). There were no nativity differences in the rate of WC increase over time among non-Mexican Hispanics. Foreign-born Mexican Hispanics also experienced a faster rate of WC increase compared to foreign-born non-Mexican Hispanics (mean difference in annual change = 0.24 cm, P = 0.01). Conclusions Longer time in the United States, examined prospectively, may only be linked to adverse anthropometric changes in some immigrant groups.