In a consectuive series of 734 patients with malignant tumorous of the lung, sarcoid reactions were observed in the mediastinal lymph nodes in 20 cases, i.e. in 3.2% of the 630 patients in whom these nodes were studied. Among these 20 patients, sarcoidosis was suspected in three. In two of these, granulomata were revealed in the tumour. Another two had metastases in the affected lymph nodes. All the common histological types of lung tumour were represented in the patients, but squamous-cell carcinomata showed a statistically significant preponderance as compared with anaplastic carcinomata. Three causes of such sarcoid reactions have been suggested: 1. an immunological reaction to substances released by the tumour and transported along the lymphatics; 2. an unrecognized sarcoidosis predisposing to lung cancer; 3. the co-existence of sarcoidosis and malignant tumour, possibly due to a common aetiological factor. The significant preponderance of squamous-cell carcinomata observed in this study is in favour of the first theory, because the slower growth and higher tendency to necrosis of this tumour type may be assumed to give rise to a more vigorous and longer-lasting stimulation of the regional lymph nodes.