Organizing pneumonia is a major reparative response of the lung tissue to an acute injury and is a pathological hallmark of an entity called bronchiolitis obliterans organizing pneumonia (BOOP). It can be idiopathic and called cryptogenic organizing pneumonia (COP) or be secondary to various conditions such as infections, drugs, connective tissue disorders, and radiation. Fifty-seven patients with pathologically confirmed BOOP were identified and were classified as having either COP or secondary BOOP on the basis of whether there was an identifiable cause. The two groups were compared for demographic, clinical, laboratory, radiological and treatment variables. Duration of treatment with corticosteroids was longer for patients with COP.