In a modified case-control study of obstructive pulmonary disease (COPD), airways obstruction has been found to be associated with age, sex, protease inhibitor type, socioeconomic status (SES) and smoking. In this paper patterns of forced expiratory flows are examined in persons demonstrating various risk factors. Two broad patterns of flow limitation emerge. The first pattern, characterized by lower flows at high lung volumes, is found in first-degree relatives of patients with COPD and subjects with a low SES. This pattern, consistent dysfunction of large airways, may reflect reversible decreases of airway caliber. The second pattern, characterized by lower flows at low lung volumes, is found in older subjects. This pattern, consistent with nonhomogeneously emptying lungs or dysfunction of small airways, may reflect more chronic irreversible changes. Smokers and male subjects exhibit both patterns of flow limitation when compared with subjects who had never smoked and female subjects. It is possible that the combination of the patterns reflects a particularly high risk for the development of COPD in male smokers.