A 61-year-old man underwent chemotherapy for a small-cell undlfferentiated lung cancer. During this treatment the primary tumour showed complete remission, whereas a lesion in the contralateral lung increased in size. This second tumour was found on cytological examination to be a moderately differentiated adenocarcinoma which was resected. Histological examination of the resected lung segment provided confirmation that this was a second primary bronchogenic carcinoma. The treatment of synchronous primary bronchogenic carcinomas is reviewed.