Chromosomal analyses of lymphocytes from lung cancer patients and normal subjects revealed that the X and the Y chromosomes have both structural and numerical abnormalities in higher frequency in patients compared to the controls. These abnormalities included chromatid/isochromatid breaks, translocations, ring formation, and selective nondisjunctions, resulting in multisomies of either the X or Y chromosomes. Possible significance of these genetic abnormalities are discussed in relation to lung cancer patients.