Though T-cell immunosenescence is a major risk factor for age-related diseases, susceptibility to infections, and responses to vaccines, differences in T-cell subset counts and representation by age and sex have not been determined for a large sample representative of the national population of the United States. We evaluated the counts of T-cell subsets including total, CD4+, and CD8+ T cells and their naïve (Tn), effector memory (Tem), and effector subsets, in the context of age, sex, and exposure to cytomegalovirus (CMV) infection among 8 848 Health and Retirement Study participants, a nationally representative study of adults older than 55 years. Total T cells (CD3+) and CD4+ cells declined markedly with age; CD8+ T cells declined somewhat less. While CD4+ T cell declines with age occurred for both CMV-seropositive and CMV-seronegative groups, total T cells and CD8+ cells were both substantially higher among the CMV-seropositive group. Numbers of Tn CD4+ and CD8+ cells were strongly and inversely related to age, were better conserved among women, and were independent of CMV seropositivity. By contrast, accumulation of the CD8+ and CD4+ Tem and effector subsets was CMV-associated. This is the first study to provide counts of T-cell subsets by age and sex in a national sample of US adults older than the age of 55 years. Understanding T-cell changes with age and sex is an important first step in determining strategies to reduce its impact on age-related diseases and susceptibility to infection.