Nitric oxide (NO) is an important mediator of inflammation in several pathological conditions. Patients with lung diseases, like asthma, have higher levels of exhaled NO (eNO) in active disease in comparison with healthy volunteers. Aspirated colonic gas in patients with ulcerative colitis (UC) showed more than 100 times higher levels of NO in comparison with normal subjects. Crohn's disease (CD) and UC are associated with a variety of systemic manifestations, although lung diseases as an extra-intestinal expression of inflammatory bowel disease (IBD) are not well investigated. In some studies, clinical and subclinical pulmonary abnormalities are described in active IBD as well as in the stable situation. The aim of the present study is to evaluate whether eNO is increased in patients with active IBD and to investigate whether there exists a correlation between (1) the eNO levels and the disease activity, and (2) the spirometry and the disease activity in a subgroup of patients. In 31 patients with CD (mean age 36.8 +/- 12.9 years) and 24 patients with UC (mean age 38.0 +/- 14.7 years) the Crohn's Disease Activity Index (CDAI) and Colitis Activity Index (CAI) were measured, respectively. Exhaled NO was measured with a chemiluminescence analyzer, according to standardized criteria. In a subgroup of CD patients, spirometry was also performed according to standardized criteria. The mean CDAI in CD patients was 192.4 +/- 94.3 and their mean eNO value was 13.5 +/- 4.6 ppb. For UC the mean CAI was 6.2 +/- 4.8 and the mean eNO value was 15.8 +/- 6.2 ppb. In a matched control group of 27 healthy, non-smoking volunteers (mean age of 33.7 +/- 13.2 years) the eNO was 10.2 +/- 2.5 ppb (P < 0.05 compared to CD and P < 0.01 compared to UC). There was a disease-activity-related increase of the eNO level in patients with IBD. For patients with UC the correlation coefficient (r = 0.63, P < 0.001) was more pronounced than for CD (r = 0.39, P < 0.05). In 17 patients with CD, spirometry was available at the time of the eNO measurement. We found a significant negative correlation between the CDAI and the FEV1 and FVC in these patients (r = -0.559, P = 0.02 and r = -0.634, P = 0.006, respectively). We conclude that eNO is increased in active IBD and correlates with the activity of the disease; furthermore, we found a negative correlation between spirometry and disease activity in patients with CD. These observations strengthen the arguments that IBD is a systemic disease. Further research is needed to try to explain the significance of an increased eNO in IBD.