Objective: The distribution of ventilation within the lung is inhomogeneous. We hypothesized that the degree of inhomogeneity in patients with cystic fibrosis (CF) differs from that in healthy subjects. Method: Three adult patients with cystic fibrosis (CF) and two healthy subjects were recruited for this preliminary study. Subjects were asked to breathe in as deep and exhale as hard as possible three times. Electrical impedance tomography (EIT) measurements were performed during spirometry tests. The global inhomogeneity index(GI) was applied to assess the degree of ventilation homogeneity at 25%, 50%, 75% and 100% of maximum inspiratory volume. Results: Airway obstruction was detected in CF patients but not in healthy volunteers during spirometry tests. GI decreased as inspiratory volume increased in CF patients (0.59+/-0.20 at 25% and 0.39+/-0.09 at 100%, p<0.01, mean+/-SD) while GI remained unchanged in healthy subjects (0.40+/-0.05 at 25% and 0.37+/-0.03 at 100%, p=0.31). Conclusion: Degree of ventilation homogeneity appears to vary during forced respiration in CF patients, but not in healthy volunteers. EIT is a practical method to measure the inhomogeneity of ventilation distribution.