Advances in operative technique and perioperative care have considerably reduced surgical morbidity and mortality after pulmonary resections. Various single and combined parameters of functional operability have been proposed to assess the surgical risk. Pulmonary function tests adequately assess the pulmonary risk, and baseline or stress electrocardiography, echocardiography and nuclear cardiac studies assess the cardiac risk. Patients with normal or only slightly impaired pulmonary function (forced expiratory volume in one second (FEV1) and transfer factor of the lung for carbon monoxide (TL,CO) > or = 80% of predicted) and no cardiovascular risk factors can undergo pulmonary resections up to a pneumonectomy without further investigation. For others, exercise testing, pulmonary split-function studies, or a combination of these two methods are recommended. Exercise testing, most frequently performed as a symptom-limited test with the measurement of maximal oxygen uptake (V'O2,max), assesses both the pulmonary and cardiovascular reserves. A V'O2,max of <10 mL.kg(-1).min(-1) is generally considered prohibitive for any resection, a value of >20 mL.kg(-1).min(-1) or >75% of predicted normal, safe for major resections. Split-function studies are radionuclide-based estimations of the predicted postoperative (ppo) values of various parameters. The currently used ppo-parameters are FEV1-ppo, TL,CO-ppo and, most recently, V'O2,max-ppo. Suggested cut-off values for safe resection are: for FEV1-ppo and TL,CO-ppo > or = 40% pred; and for V'O2,max > or = 35% pred, combined with an absolute value of > or = 10 mL.kg(-1).min(-1). The lowest acceptable ppo-values will still have to be established by additional prospective studies. In the long-term, resections involving not more than one lobe usually lead to an early functional deficit followed by later recovery. The permanent functional loss in pulmonary function is small (< or = 10%) and exercise capacity is only slightly reduced or not at all. Pneumonectomy, on the other hand, leads to an early permanent loss of about 33% in pulmonary function and 20% in exercise capacity. Thus, pulmonary function tests alone overestimate the functional loss after lung resection.