AN enumeration of new cases of lung cancer (i.e. cancer of the trachea, pleura, lungs or bronchi) diagnosed in 3 hospital centres in Great Britain during the years 1948-1952 was made previously (Bonser and Thomas, 1955). It was noted that more deaths were recorded than cases were diagnosed in the hospitals in two regions-Aberdeen and Leeds City. However, the figures were not strictly comparable, as not all the cases identified clinically in any year would die in the same year, though the overlap would tend to balance over the five-year period. In the Aberdeen region, approximately one-fifth of recorded deaths from lung cancer did not appear in the hospital records; in Leeds City one-tenth. But in the latter area, there was a greater discrepancy between recorded female deaths and female cases diagnosed in hospital, approximately one-third of recorded female deaths not appearing in the hospital records. The reason for this was obscure unless it happened that more deaths from secondary tumours of the lung were included in the female deaths.The present investigation was undertaken in order to obtain more accurate information about the relation between the numbers of recorded deaths from lung cancer and the numbers of cases diagnosed clinically. Leeds is a city of approximately half a million inhabitants, with two large general hospitals.