Malignant disease of the lungs is considered to be such a rarity, that it is usually given no more than a paragraph in our text books and is consequently seldom brought up in discussion of differential diagnosis. As long ago as 1889, Hare estimated that of all the deaths from pulmonary disease in London, 2·3 per cent, were due to primary neoplasms. If this be true, and if malignant disease of the lungs has increased since then in the same proportion as it appears to have increased in other organs, this figure should now be much larger. Unfortunately, as the diagnosis is so difficult, and our tacilities for postmortem examinations are so unsatisfactory, it has not yet been possible to verify this.