In a previous paper (Doll and Hill, 1950) we reported the first results of a large-scale investigation undertaken to determine whether patients with carcinoma of the lung differed materially from other persons, either in their smoking habits or in some way which might be related to the theory that atmospheric pollution is responsible for the development of the disease. We concluded that smoking is a factor in the production of carcinoma of the lung, and this conclusion was in conformity with the results of some other investigations. Our first observations were, however, limited to patients drawn mainly from London and the adjacent counties. We have now extended the investigation to other parts of the country and have made more detailed inquiries into smoking habits. Many further patients have been interviewed (during January, 1950, to February, 1952) in hospitals in Bristol, Cambridge, Leeds, and Newcastle-upon-Tyne, and also in eight of the twenty London hospitals which co-operated in the first part of the inquiry.