Evidence that host immunologic function may influence the behavior of lung cancer is accumulating. Non-small-cell lung cancers are heavily infiltrated by host lymphocytes. The fact that monoclonal antibodies have been developed against lung cancer cells implies that such cells express surface antigens and are therefore vulnerable to immune recognition. Failure of the host defense mechanism to control tumor growth may involve (1) reduced natural killer cell activity, (2) inadequate lymphokine-activated killer cell function, or (3) tumor secretion of immunomodulating factors. Basic immunologic research studies of lung cancer are increasing the potential for clinical applications. New monoclonal antibodies have improved both the sensitivity and the specificity of immunohistopathologic analyses of pulmonary specimens. Links between immune function and prognosis in patients with lung cancer have been established. Finally, initial results from protocols that have used tumor-infiltrating lymphocytes, interleukin 2, and tumor vaccines suggest that immunobiologic treatment modalities may be increasingly applicable in patients with lung cancer.