Eleven cases of clinically diagnosed cryptogenic organizing pneumonia were examined in order to establish the histological features found at transbronchial biopsy and to correlate this with open lung biopsy which followed in six cases. The essential pathological feature was the presence of buds of granulation tissue (Masson bodies) indicating organization of a persistent exudate by fibroblasts and capillaries within alveoli, with preservation of the alveolar architecture. In addition, both acute and chronic inflammatory cells were present in the interstitium. These features, combined with the clinical history, were sufficient for a diagnosis in seven of the 11 cases on transbronchial biopsies. Four biopsies lacked these features. Patients proceeded to open lung biopsy in addition to the two with histological features of cryptogenic organizing pneumonia on transbronchial biopsy, but where the clinician wanted to eliminate other pathology because of rapid clinical deterioration. Five of the six cases coming to open lung biopsy confirmed cryptogenic organizing pneumonia with Masson bodies within alveoli, but changes were focal in three with very few Masson bodies in one. One case which had the features on transbronchial biopsy lacked them in the open lung biopsy. Transbronchial biopsy, therefore, can yield diagnostic material in the majority of patients with cryptogenic organizing pneumonia while open lung biopsy, which is considered the gold standard for interstitial lung disease, may yield negative results because of sampling error and the rapid evolution and changing pattern of the disease.