Subjects with severe chronic obstructive pulmonary disease (COPD) may have marked differences in emphysema severity on chest computed tomography (CT) scans. Although many patients with severe COPD will have chest CTs performed during their clinical care, chest CTs have not been widely included in epidemiologic and genetic studies of COPD. We sought to determine whether chest CT scans performed for clinical indications can provide useful data in an epidemiologic study of COPD and to determine whether chest CT scans can be used to define subtypes of severe, early-onset COPD. Clinical chest CT scans on 91 probands in the Boston Early-Onset COPD Study were retrospectively reviewed by 2 pulmonologists and 1 to 2 chest radiologists, using a semi-quantitative emphysema severity score, ranging from 0-24. 88 of 91 chest CT scans were suitable for emphysema analysis. There was a wide range of emphysema severity, from mild to severe (1.3-23.7). Emphysema-predominant subjects (upper 3 quartiles of emphysema scores) had more severe airflow obstruction than airway-predominant subjects (lowest quartile of emphysema scores): FEV(1) 17.4% vs. 22.4% predicted, p=0.009. A higher percentage of airway-predominant subjects had a positive bronchodilator response (28.6% vs. 6.7%, p=0.009). Airway-predominant subjects also had a higher frequency of physician-diagnosed asthma (p=0.04) and a trend towards higher serum immunoglobulin E levels (p=0.09). Analysis of siblings of early-onset COPD probands suggested a genetic contribution to the subgroups. Using clinical chest CT scans, we were able to identify an airway-predominant subgroup with asthma-like features among subjects with severe, early-onset COPD.