It has been reported that in patients with inflammatory bowel disease (IBD), the airways are involved, and a number of clinical manifestations have been described. The aim of this study was to investigate the function of the small airways in IBD. Thirty patients with IBD (mean age, 47 yr), 12 with Crohn's disease and 18 with ulcerative colitis, were studied and compared with a control group of 16 normal subjects. Maximal expiratory flow-volume curves were performed breathing room air and a mixture of 80% helium, 20% oxygen. The differences of flows at 50% of FVC (delta Vmax50) and the volume of equal flows (Visov) were calculated as indices of small airways function. In addition, spirometry, lung volumes, and diffusing capacity were measured. Visov was statistically significantly greater in patients with either CD or UC than in control subjects (x +/- SD) (24.99 +/- 1.35 and 25.95 +/- 1.5 versus 20.1 +/- 1.39), (p < 0.01 and p < 0.001, respectively). A reduction in TL(CO) was noticed in the active stage of the disease in both groups of patients (p < 0.05). This may indicate that lung parenchyma is also involved in active IBD. Our results suggest that the function of the small airways and diffusion capacity of the lungs are affected in patients with IBD.