We evaluated the alveolar-arterial oxygen difference (deltaA-a) and the ratio between PaO2 and the fractional concentration of inspired oxygen (P/F) in acute pancreatitis. Eleven patients had mild uncomplicated disease, six showed acute abdominal fluid collections, six had acute abdominal collections and asymptomatic x-ray lung involvement, three presented transient dyspneic episodes, and four had severe acute pancreatitis requiring prolonged oxygen therapy. In the uncomplicated disease, respiratory function was normal; in the six patients with abdominal collections only, deltaA-a increased by 50% and P/F decreased by 20-30%; in the six patients with abdominal collections and asymptomatic x-ray lung involvement, deltaA-a increased by 50-70% and P/F decreased by 40%; the three patients with dyspneic episodes showed a twofold increase in deltaA-a and a 40% decrease in P/F; the four patients with severe pancreatitis had a two- to threefold increase in deltaA-a and a 40-50% decrease in P/F. Hence respiratory function is normal only in uncomplicated pancreatitis; in the presence of complications, disturbance of gas exchange always occurs, requiring careful control and treatment.