High-resolution computed tomography (CT) was correlated with pulmonary function tests in the evaluation of regional emphysema in 59 smokers. The lung was divided into upper (above the carina tracheae) and lower (below the carina tracheae) zones, and the degree of emphysema was graded with a subjective and an objective measurement. Functional emphysema was defined as a diffusion capacity less than 75% of predicted and forced expiratory volume in 1 second less than 80% of predicted. Three of 15 (20%) subjects with functional emphysema had no subjective evidence of emphysema at high-resolution CT, and 10 of 25 (40%) with emphysema at high-resolution CT had no functional abnormalities consistent with emphysema. Even though the upper lung zones were more severely affected by emphysema, the degree of emphysema in the lower zones had a stronger correlation with pulmonary function abnormalities. The upper lung zones are a relatively silent region where extensive destruction may occur before functional abnormalities become known.