Immigrants have been discussed as a means of alleviating fiscal pressures on Social Security. Their long-term impact on the Social Security system depends critically on their fertility and mortality patterns. In this paper, we examine the fertility and mortality patterns of immigrants to the United States and compare these patterns with those of non-immigrants. We find that both the recent and cumulative fertility of immigrant women is higher than that of native-born women, but that a large share of these differentials can be “explained” by differences in age structures, race and ethnicity, years in the United States, and country of origin. Using a synthetic cohort approach, we examine the role of years in the United States in more detail, and find no evidence of assimilation towards native-born fertility patterns. Consistent with previous research, we find evidence of a disruption effect on fertility – the fertility of immigrant women in the most recent arrival cohorts is low, but increases at a faster rate relative to both the fertility of immigrants from earlier cohorts and relative to the fertility of natives. We find that immigrants experience lower mortality than native-born individuals in the United States, and these differences remain even after controlling for underlying differences in observable characteristics. However we find that they do not exhibit differences in their subjective expectations of their mortality.