Cryptogenic organizing pneumonia (COP) or bronchiolitis obliterans organizing pneumonia (BOOP) designates a disease characterized by particular histopathological features. Symptoms and clinical findings are more or less typical and the etiology of the disease is unknown in most of the cases. We report on 7 patients in whom the diagnosis was established by open lung biopsy during a workup for persistent pulmonary infiltrates. The clinical findings, the radiological features and the course during treatment with corticosteroids are analyzed. In all patients the symptoms improved. In two cases we observed relapses after discontinuing the corticosteroids or diminishing the dose. These relapses again responded to steroids, but in one patient progressive pulmonary restriction was noticed despite continuous treatment.