['Emphysema, especially in the upper lobes, is frequently observed in association with idiopathic pulmonary fibrosis (IPF). However, the combination of emphysema plus IPF has received little attention.', 'To investigate the additional functional impairment from emphysema in IPF patients.', 'Twenty-one patients (mean age 66 y, 20 men) (Group I) who had both IPF (mean 35% of total lung volume) and emphysema (mean 14% of total lung volume) were compared to a group of 21 subjects who had IPF but no emphysema (Group II). The groups were matched for (among other criteria) the total extent of disease. Pulmonary function tests, Medical Research Council dyspnea score, 6-min walk test, and radiographic extents of both IPF and emphysema were obtained for each patient. The Composite Physiologic Index was calculated. In the total population (n = 42), the independent contributions of IPF and emphysema to several physiologic variables were investigated by using stepwise multiple regression analysis.', 'Despite the limited extent of emphysema, Groups I and II had similar physiologic impairment. Only residual volume and total lung capacity were significantly higher in Group I. According to stepwise multiple regression analysis, the extent of IPF and either the presence or the extent of emphysema in the total population were independent and significant predictors of dyspnea score, 6-min walk test, P(aO2), forced expiratory volume in the first second (FEV(1)), forced vital capacity (FVC), FEV1/FVC, the diffusing capacity of the lung for carbon monoxide, carbon monoxide diffusing capacity adjusted for alveolar volume (gas-transfer coefficient), and residual volume. The Composite Physiologic Index was closely related to the extent of IPF (r = 0.65, p < 0.0001) and to the dyspnea score (rho = 0.59, p < 0.0001).'] Does the presence of emphysema further impair physiologic function in patients with idiopathic pulmonary fibrosis?