Of 150 consecutive patients with resectable bronchogenic carcinoma, 11 were found to have multiple primary tumours of the respiratory tract. Five patients had more than one primary tumour of the lung and six had previously received treatment for carcinoma of the larynx. In the same group of 150 patients, 14 had one or more cancers involving other organs. The appearance of a new solitary lung lesion in a patient with another cancer demands special attention to determine the exact diagnosis and to decide upon the best method of treatment. Physicians should be aware that such a lung lesion may represent a new primary lung cancer rather than a metastasis. The criteria are defined for the pathological definition of two separate primary tumours in these cases.