Immigration is an important population dynamic at work in the U.S., but we know little about its impact on American obesity. Built on nutrition transition and immigration theories, this paper provides explanations for immigrants’ initial body composition advantage, its partial erosion over time, and the gender difference in the erosion. We find evidence that the American obesity epidemic would be much more severe without the mass immigration that began in 1965. In addition to confirming the erosion in immigrants’ body composition advantage, we further find that this erosion is weaker for men than for women. Once immigration’s impact is teased out, racial/ethnic disparities in body composition greatly differ from what we observe. This study provides gender-specific estimates for the differences in obesity by nativity and residence duration and the net level of Hispanic-white and Asian-white disparities at the mean body mass index (BMI) as well as the overweight, Stage-1, and Stage-2 obesity cutoffs. Our findings suggest that immigration must be taken into account when addressing public health concerns.