We identified a total of 199 patients with coexisting restrictive and obstructive lung disease. Of these patients, 37% were women and 74% were white (Table 1). The mean age was 56 ± 14 years; the mean FEV1, 1.5 ± 0.6 L (42 ± 14% predicted); mean FEV1/FVC, 0.61 ± 0.06; and mean TLC, 4.5 ± 1 L (71 ± 7% predicted). Based on referral requests, patients had a variety of lung diseases, which were categorized into obstructive diseases (eg, asthma, COPD, bronchiectasis), interstitial lung diseases (eg, IPF, pulmonary edema), and extrapulmonary restriction (eg, kyphoscoliosis, obesity). Other categories included lung cancer; evaluation for heart, lung, liver, or kidney transplantation; and unexplained dyspnea. More than one-half were current or former smokers, and many such smokers presented with airflow obstruction (and restriction) diagnosed based on PFTs without having had a previous diagnosis of asthma, COPD, or other obstructive lung disease. The correlation between FEV1 and RV/TLC was significantly greater when using the adjusted than when using the unadjusted FEV1 (r2 = 0.42 vs r2 = 0.36, respectively; P = .01) (Fig 1), suggesting that the adjusted value more accurately reflects the degree of obstruction. By ATS/ERS grading, the unadjusted data categorized 76% of the patients as having severe or very severe obstructive disease, with 11% having mild or moderate disease (the remainder having moderately severe disease). The adjusted data set rated 33% with severe or very severe disease and 45% with mild or moderate disease; this distribution was significantly different from that using the unadjusted values (P < .0001) (Fig 2). In comparison, by ITS guidelines, 98% were graded in the mild to moderate range, with < 1% in the severe category. Among the adjusted values, 83% of patients had a change in grading to less severe obstruction by ATS/ERS guidelines, and 44% and 70%, respectively, had a change to less severe obstruction based on the asthma and COPD severity grading systems (Fig 3). See e-Appendix 1 for the results based on adjusting the FEV1 by the more accurate factor of 0.93.