The repeated finding of malignant or suspicious cells in the sputum of an apparently normal person must not be ignored. Immediate and intensive attempts should be made to localize the carcinoma. Sometimes, in the presence of a central tumour an early diagnosis can be made radiologically. With peripheral tumours and early diagnosis is more likely. The most successful method for the early localisation of an occult bronchial carcinoma is fibre optic bronchoscopy, with selective bronchial aspiration. Consistent finding of malignant cells in the secretions from a lobar bronchus, provided contamination can be excluded, justifies surgery such as a lobectomy. Delay in instituting treatment while waiting for radiological or bronchoscopic confirmation reduces the chance of successful surgery because of the possibility of metastases or the development of cardiological or respiratory complications.