The aim of this study was to investigate lung function in patients with ulcerative colitis and to assess the incidence of latent pulmonary involvement in subjects with active and inactive disease. After full colonscopic assessment with multiple mucosal biopsy, the clinical disease activity of each patient was quantified, using the simple index of Harvey and Bradshaw. The patients were divided into 2 equal groups: subjects with active disease (group 1; n=16); and those with inactive disease (group 2; n=16). Global spirometry was then performed. A latent pulmonary involvement was found in 17 of 32 patients (53%), the incidence was higher in the group 1 patients (81%). The majority of patients presented a reduction in the carbon monoxide diffusing capacity of the lungs (DL,CO). The mean DL,CO value was 73.87+/-14.87 in group 1 and 87.31+/-11.23 in group 2. The DL,CO and KCO reduction correlated significantly with intestinal histopathological grading in the group of patients with active disease (r=0.87, p<0.001; r=0.603, p=0.015). To conclude, a high incidence of pulmonary function abnormalities were identified, despite the lack of radiological alterations (High Resolution Computed Tomography) and pulmonary symptoms, in ulcerative colitis patients. These alterations were more common in patients with active disease. The strong correlation between DL,CO values and histopathological grading suggests that this test may reflect bowel disease activity.