Summary: Sixty-nine patients with a chest radiograph showing enlargement of the hilar and/or paratracheal lymph nodes were analysed. Ten of these, mainly Asian, were found to be suffering from tuberculosis. Of the remaining fifty-nine, who had sarcoidosis, 12% were tuberculin-positive. All the cases diagnosed as sarcoidosis had either a positive Kveim test and/or mediastinal lymph node histology compatible with sarcoidosis. 10% of cases with sarcoidosis showed evidence of uveitis and 10% showed bone cysts and there was also a high proportion of other extrathoracic lesions. From these studies we conclude that, whilst the currently accepted radiological staging of sarcoidosis serves as a convenient form of reference, these stages do not in any way relate to the clinical stage of disease, or to the histological appearance of affected lymph node tissue. There is a correlation between a strongly positive Kveim test of three or more granulomas and mediastinal lymph nodes showing confluent granulomata (predominantly of the epithelioid cell-type) with fibrosis. The presence of intra- or peri-granulomatous fibrosis, or even gross diffuse fibrosis, does not seem in itself to be related to the nature of onset, clinical or accepted radiological staging of disease. However the mediastinal lymph nodes in patients with erythema nodosum were predominantly of giant cell-type; these patients were also Kveim-positive. Similar instances of giant cell predominance in the mediastinal lymph nodes were seen in each of the groups examined. Further studies are required to determine whether these cellular characteristics, or other histological features in the mediastinal lymph nodes, might be of prognostic significance.