Pulmonary function tests and tracheal cross-sectional area (X-SA) measured on chest roentgenograms were analyzed in 72 healthy pairs of twins and in 34 healthy sons of patients with chronic obstructive lung disease. In the twin study, genetic influence was positive on most pulmonary function tests, including VC, FEV1/FVC, V25, but not so on X-SA. The ratio of X-SA to VC, which represents dysanapsis, had no relation to sex nor height. However, the ratio of X-SA/VC showed marked sex-associated differences, when expressed as a function of VC, and had a negative correlation with VC both in men and women. The ratio of X-SA/VC was significantly lower in sons of patients with COPD than in normal controls, which was caused by significantly larger VC in the former group. The relatively low ratio of tracheal size to lung size may partly explain the susceptibility to the possible future development of COPD in those subjects.