A 29 year-old-man with Crohn's disease, who developed diffuse pulmonary infiltrates and hypoxemia two months following oral administration of mesalazine, was examined. Clinical findings and computed tomography were suggestive of, and lung histology was diagnostic of, bronchiolitis obliterans organizing pneumonia, also known as cryptogenic organizing pneumonia. Although the data did not allow for definitive conclusions, they did suggest that the pulmonary disease was an extraintestinal manifestation of Crohn's disease, rather than an adverse reaction to mesalazine. In fact, the patient showed clinical, radiological and functional improvements, despite the treatment with mesalazine and the withdrawal of steroid therapy.