After influenza challenge, aged mice have prolonged viral shedding that correlates with lower splenic cytotoxic T lymphocyte (CTL) activity. To evaluate the age-related pulmonary cell-mediated immune response to influenza, pulmonary lymphocytes were obtained from young and aged mice at various days after respiratory tract infection with nonlethal influenza A/PC/1/73 (H3N2) virus. In young mice, pulmonary CTL activity peaked at 48% +/- 2% on day 7 after infection. Pulmonary CTL activity peaked 1 day later in aged mice and at about half the activity (24% +/- 5%). The majority of the cells recovered from the lungs in both age groups were CD3+, CD8+ T cells. Histologic examination of the lungs revealed that aged mice had significantly less inflammation than young mice. Therefore, after influenza challenge there was a large influx of lymphocytes into the lungs of both young and aged mice, but the cells from young mice were more active on a per-cell basis. In a further experiment, challenge with a more virulent strain of influenza produced higher mortality in young mice than in aged mice. Thus the higher CTL activity of young animals leads to more rapid virai clearance, but this may be at a price to the host--that is, more immunopathologic damage.