Cryptogenic organizing pneumonia (COP), also known as bronchiolitis obliterans organizing pneumonia (BOOP), is an uncommon lung disease characterized by the presence of granulation tissue within the alveolar ducts and alveoli. Because of the limited published literature on this topic and limited information on outcome we reviewed our own experience over an 8-year period and also critically evaluated the literature. We reviewed all cases of COP diagnosed from 1985 through 1992 at Vancouver General Hospital: 25 patients (14 male, 11 female) aged 20-77 years (mean, 49 yr, SD +/- 17 yr). Nine patients had myeloproliferative disorder, including 6 who had allogenic bone marrow transplants; 2 patients had connective tissue disease; and 14 patients had no underlying disease (idiopathic). Data retrieved retrospectively from clinical records included demographics, risk factors, symptoms, chest radiographs, computerized tomograms, lung function tests, therapy prescribed, and response to therapy. Symptoms included dyspnea and cough (n = 15) (60%), cough only (n = 10) (40%), and fever (n = 15) (60%). Twenty-two patients were diagnosed by open lung biopsy and 3 by transbronchial biopsy. Lung imaging showed bilateral patchy airspace consolidation or nodular opacities as the main finding in 22 patients. Pulmonary function tests showed a combined restrictive and obstructive pattern. All patients received prednisone therapy except 1 patient whose idiopathic findings resolved completely with minimal treatment. Eight patients died, including 4 of the 9 patients with myeloproliferative disorder--2 from a combination of respiratory failure due to COP and graft-versus-host disease. One of 2 patients with connective tissue disease died, and 3 of 14 patients with idiopathic COP died. COP is an uncommon condition but should be considered in patients with bilateral airspace disease, especially those who fail to respond to antibiotics for presumed pneumonia. Although pulmonary function tests and CT scan findings in conjunction with the clinical features usually suggest the diagnosis, definite confirmation usually requires either open lung biopsy or transbronchial biopsy. Histologic confirmation of the diagnosis is particularly warranted as therapy with corticosteroids is usually needed for a number of months. The prognosis is excellent with idiopathic cases but more guarded especially when COP is associated with lymphoproliferative or connective tissue disease.