Toxoplasma gondii pneumonia has emerged as an important problem in immunocompromised patients, especially those with AIDS. The characteristics of lung lymphocytes, including their phenotypes, proliferative responses, and suppressor function during T. gondii pneumonia were studied. During primary acute T. gondii infection, the numbers of T lymphocytes in the lungs increased even though mice lacked histologic evidence of pneumonia. As mice recovered from acute toxoplasmosis, numbers of lung CD4+ cells and the ratio of CD4+ to CD8+ cells decreased. Subsequently, T. gondii infection reactivated, manifested as pneumonia. During pneumonia, lung T lymphocytes, especially CD8+ cells, increased even more. Lung lymphocytes from mice with T. gondii pneumonia decreased the proliferative responses of splenocytes from T. gondii-immune mice to both concanavalin A and T. gondii lysate antigens in vitro. The striking increase in lung CD8+ cells and suppressor activity appear to be integral to the pathogenesis of T. gondii pneumonia.