Results are reported of a prospective study, carried out by the Mass Radiography Service of the North-West Metropolitan Region for the purpose of evaluating early lung cancer detection by six-monthly chest radiographs. The lung cancer experience of a test group of 29,723 men aged 40 and over who were offered six-monthly chest radiographs over a period of three years is compared with a similarly constituted control group of 25,311 men who were radiographed only at the beginning and the end of the study. In the test group 29,416 men (98·9%) and in the control group 25,044 men (99%) were followed up. The methods employed to achieve this result are analysed. The six-monthly surveys of the test group yielded 65 cases of lung cancer, giving an annual incidence and detection rate of 0·9 per thousand examined. Of these cases 65% were resected. Of all cases of lung cancer in the test group, irrespective of their source of detection, 43·6% were operable, compared with 29% in the control group. The difference (P=0·03) is statistically significant. The annual mortality rate from lung cancer based on 62 deaths in the test group and 59 deaths in the control was 0·7 and 0·8 per thousand respectively. The conclusions are reached that since early detection by six-monthly chest radiographs has not significantly reduced the mortality from lung cancer in a population at risk, a policy of such large-scale surveys of men in the cancer age would not seem justified, but that the increased discovery of resectable lung cancer by this method forms a reasonable basis for encouraging individuals in high-risk groups to make regular use of existing mass radiography facilities.