If U.S. birthrates remain below the replacement level and are relatively constant, and if immigration is constant both in total numbers and age-sex composition, population size will eventually become stationary, according to this policy discussion paper. Moverover, varying the constant annual number of immigrants produces an equal proportionate change in the size of the resulting stationary population. A third, often overlooked factor that can affect stationary population size is changes in immigrants' ages. Projections of the 1980 population that assume the admission of 560,000 U.S. immigrants each year show that the ultimate stationary population contains 14.4 million people if immigrants are admitted only at ages 50 to 54. But when all immigrants are admitted at ages 10-14, the resulting stationary population is 328.3 million. Thus, age at admission makes a large demographic difference--in this case, a size difference of almost 23 to 1. The intuitive reason for the large difference is that 50-year-old migrants are too old to contribute descendants born in their new country. Teenage migrants, on the other hand, contribute not only themselves but also descendants, and their descendants will in turn further reproduce. To the degree that national policy wants to use immigration to build up population size or allow it to decrease, policy could take more account of immigrants' ages. Data are presented in the text and on tables and graphs. List of references and related titles are appended. (Author/KH)