For decades, surgery has been the mainstay of treatment in early stage lung cancer, as well as complex infectious conditions of the lung. Concomitantly, forced expiratory volume (FEV₁) alone has been the discriminating parameter of surgical eligibility. Still, the limitations of FEV₁ have emerged in published literature, and so carbon monoxide diffusing capacity (DLCO) has been suggested as a complementary parameter, to be introduced along with the concept of defining numeric values corresponding to the predicted postoperative figures of FEV₁ and DLCO. However, recent evidence argues against the predicting ability of postoperative FEV₁ and DLCO, especially in the immediate postoperative period and in patients with severe airflow limitation.