OBJECTIVE The prediction of postoperative preserved pulmonary function is essential for ascertaining the functional operability of pulmonary metastasectomy candidates. Formulae to predict pulmonary function after metastasectomy have not yet been described. This study was undertaken to provide data about the functional loss after a pulmonary metastasectomy, which often includes non-anatomical resections or combinations with anatomical resections. METHODS Pulmonary function tests were performed preoperatively, postoperatively and 3 months after a pulmonary metastasectomy, and the factors potentially influencing the functional outcome were prospectively collected in a database. The functional loss was calculated as the difference in the values between the follow-up visit and the preoperative values, and the influencing factors were tested using the Mann-Whitney test. RESULTS A total of 162 patients were prospectively included in the study and 117 completed the study protocol with a follow-up evaluation after a mean of 3.4 months. Of these, 33 patients had bilateral resections, 30 interventions were repeated resections and adhesions were removed in 46. The greatest lung resection performed was a lobectomy in 13, with segmentectomy in 27 and wedge resection in 77 patients. The mean overall functional loss was: forced vital capacity -9.2%, total lung capacity -8.8%, forced expiratory volume in 1 s -10.8% and diffusion capacity for carbon monoxide (DLCO) -9.7%, whereas the diffusion coefficient (KCO) and pO(2) remained unchanged after 3 months. This functional loss was significant (P < 0.001) for all the parameters mentioned. The two factors were inversely found to influence the functional outcome: bilateral resection reduced spirometry values (P < 0.01), postoperative chemotherapy reduced DLCO (P = 0.011) and KCO (P = 0.029). CONCLUSIONS A pulmonary metastasectomy leads to a significant loss of pulmonary function after 3 months in an average patient collective. The most important factors for deteriorating lung function are a bilateral operation and postoperative chemotherapy.