This paper aims to address a gap in our understanding of immigrant health issues by examining the determinants of excess weight--an important indicator of current and future health. The paper combines data drawn from recent large health surveys to identify how the weight of recent immigrants compares with that of native-born people, and how the likelihood of becoming overweight or obese changes with additional years in Canada. We find evidence that on average, immigrants are substantially less likely to be obese or overweight upon arrival in Canada. These measures converge slowly to native-born levels, but there is marked variation by the ethnicity of the immigrant. Since changes in weight will reflect choices with respect to diet and activity, the extent to which overweight and obesity rates change with years in Canada may reflect the extent to which immigrants interact with or are influenced by members of their ethnic group who reside in the same area. We find evidence that ethnic group social network effects exert a quantitatively important influence on the incidence of being overweight and obese for members of most ethnic minorities, tempering the process of adjustment to Canadian lifestyle norms that may be driving excess weight gain with additional years in Canada.