Background: The percent predicted diffusing capacity of the lung for carbon monoxide (Dlco%) is an important pulmonary function test (PFT) obtained before elective pulmonary resection. However, there are several Dlco values reported and it is unknown which ones are important predictors of postoperative morbidity. Methods: This is a retrospective study of a prospective database of patients who underwent PFTs and pulmonary resection by one surgeon. The PFTs evaluated were as follows: forced expiratory volume in one second (FEV1%), minute ventilation volume (MVV%), and three types of diffusion capacity of the lung for carbon monoxide values: the diffusion capacity of the lung for carbon monoxide (Dlco%), the Dlco adjusted for hemoglobin (DL adjusted%), and the Dlco adjusted for alveolar volume (Dlco/VA%). Results: There were 906 patients between January 2005 and December 2007, and lobectomy was performed most commonly. Complications occurred in 254 patients (28%) and were respiratory in 115 (13%). On univariate analysis, age (p < 0.001), number of cigarettes smoked (p = 0.008), history of coronary artery disease (p = 0.028), FEV1% (p = 0.021), postoperative predicted (ppo) FEV1% (p < 0.001), Dlco% (p = 0.018), ppoDlco% (p = 0.002), and Dlco/VA% (p = 0.004) were significantly different among those who did and did not experience postoperative respiratory morbidity. Multivariate regression analysis identified ppoDlco%, ppoFEV1%, Dlco/VA%, and age as significant independent predictors of respiratory morbidity. Operative mortality was 2% (18 patients). Conclusions: Although age, FEV1%, ppoFEV1%, Dlco%, and ppoDlco% are all well-known predictors of operative morbidity after elective pulmonary resection, the Dlco/VA% is another important predictor. This information should be included to help guide patient selection for pulmonary resection and to determine preoperative risk stratification. © 2009 The Society of Thoracic Surgeons.