The aim of the present study was to investigate changes in pulmonary function tests (PFTs) in patients with acute pancreatitis (AP), to compare them with those changes in healthy controls, and to analyze the relationship between these parameters and computed tomography severity index (CTSI) and Ranson's criteria scores as markers of disease severity. This study included 40 patients with AP without a diagnosis of any pulmonary disease and 40 sex- and age-matched healthy controls. All participants were evaluated with simple PFTs and single-breath carbon monoxide (CO) diffusion tests. Patients with AP were also evaluated according to their CTSI and Ranson's criteria scores as markers of disease severity. The forced vital capacity (FVC), forced expiratory volume in the first second (FEV1), FEV1/FVC, and peak expiratory flow, which determine lung capacity, were similar in the two groups. The forced expiratory volume during the middle half of the FVC (FEF25 - 75%), CO diffusing capacity (D-LCO), and ratio of D-LCO to alveolar ventilation (D-LCO/V-A), which determines alveolar membrane permeability, revealed a statistically significant decline in pulmonary gas exchange in patients with AP (P < 0.05). Correlation analysis showed that there is a significant negative relationship between CTSI and Ranson's criteria scores with FEF25 - 75%, D-LCO, and D-LCO/V-A (P < 0.05). We suggest that AP may cause impaired alveolar gas exchange without manifest pulmonary diseases. The effect of AP on FEF25 - 75%, D-LCO, and D-LCO/V-A appears to be dependent not only on the disease, but also on its severity. FEF25 - 75%, D-LCO, and D-LCO/V-A may give additional prognostic information in patients with AP in the initial evaluation.