Background: We sought to determine if the rate of increase in body mass index (BMI) differs between first generation immigrant children (child and both parents born outside Canada); second generation immigrant children (child born in Canada with at least one parent born outside Canada); and native-born children (child and both parents born in Canada), and if the rate of increase varies across ethnic groups.Methods: Data were available from the evaluation of a 5-year heart health promotion program targeted to elementary school children from 24 schools in multi-ethnic, disadvantaged, inner-city neighborhoods in Montreal, Canada. Participants were 6392 children aged 9–12 years born in and outside of Canada. Height and weight were measured annually according to a standardized protocol. BMI increases with age were examined using individual growth models stratified by immigrant status grouping (first generation immigrant, second generation immigrant, native-born).Results: On average, BMI increased by 0.59, 0.73, and 0.82 kg/m2 with each year of age among first generation immigrant, second generation immigrant, and native-born children, respectively. These differences held across four family origin grouping (Europe, Asia, Central/South America, and Other).Conclusion: The protective effect of immigrant status on BMI increases with age dissipated in second generation immigrant children, whose rate of increase was similar to that of native-born children. Because immigrants constitute the fastest growing segment of the Canadian population, it is important to understand the causes of the higher BMI increases with successive generations.