Crohn's disease is a granulomatous systemic disorder of unknown aetiology. Obvious pulmonary involvement is exceptional. We report the case of a 33 year old woman treated with mesalazine for Crohn's disease and presenting with dyspnoea. Pulmonary function tests showed a restrictive ventilatory pattern with hypoxaemia on exertion. Chest radiography disclosed an interstitial pattern with ground glass on high resolution computer tomography. Clinical and radiological abnormalities progressed after withdrawal of mesalazine. Corticosteroids led to a partial regression but were stopped because of severe side-effects. Because of worsening of the clinical situation, open lung biopsy was performed and showed a histopathological aspect of nongranulomatous interstitial diffuse lung disease with an inflammatory lymphoid infiltration associated to some mild interstitial collagen fibrosis. Addition of cyclophosphamide to high-dose pulse steroid therapy induced a significant and sustained improvement.