Cryptogenic organizing pneumonitis is a clinical and pathologic syndrome characterized by a "pneumonia-like" illness with excessive proliferation of granulation tissue within small airways and alveolar ducts associated with chronic inflammation in the surrounding alveoli. The duration of illness prior to lung biopsy is short, usually less than 2 months, and it is markedly different from that of IPF. Interestingly, unlike in IPF where the patient has difficulty remembering the exact onset of symptoms, patients with COP are frequently very specific about the timing of their disease onset. This is because the disease onset is recent and is often dramatic with the development of a severe flulike illness, ie, cough, fever, malaise, fatigue, and weight loss. Inspiratory crackles are frequently present on chest examination. Pulmonary function is usually impaired with a restrictive defect being most common. Gas exchange abnormalities are extremely common with a reduction in Dco and resting hypoxemia being almost universal findings. The roentgenographic manifestations are quite distinctive with a pattern of bilateral, diffuse but inhomogeneous, ground-glass or alveolar opacities being present in the majority of the cases. Bronchoalveolar lavage findings are nonspecific but usually reveal a lymphocytosis. The response to corticosteroid treatment is quite favorable and death from progressive disease is uncommon in COP, especially if treatment is instituted early in the course of the disease. In our experience, the cases with the worse prognosis are those associated with another disease process, in particular, connective tissue disorders like rheumatoid arthritis. In fact, these patients are prone to develop a rapidly progressive form of BOOP with a clinical course similar to the "Hamman-Rich syndrome." Recurrences are relatively frequent, consequently, withdrawal of treatment should be done with extreme caution. Corticosteroids have been the conventional initial treatment of COP, although to our knowledge, there are no controlled clinical trials to support it use. Antibiotics are not effective in treating this syndrome. Thus, based solely on our experience and that of others, we believe that high-dose corticosteroid therapy should be used to treat COP, usually initiated with 1 to 1.5 mg/kg/day (using ideal body weight) not to exceed 100 mg/day. Prednisone is given as a single oral dose in the morning. We recommended maintaining this dose for 4 to 8 weeks. If the patient's condition is stable or improved, the prednisone dosage is gradually tapered to 0.5 to 1 mg/kg/day (using ideal body weight) for the ensuing 4 to 6 weeks.(ABSTRACT TRUNCATED AT 400 WORDS)