In the last five years a number of studies have been made of the smoking habitsof patients with and without lung cancer (Doll and Hill, 1950, 1952; Levin, Gold--stein, and Gerhardt, 1950; Mills and Porter, 1950; Schrek, Baker, Ballard, and Dolgoff, 1950; Wynder and Graham, 1950; McConnell, Gordon, and Jones, 1952; Koulumies, 1953; Sadowsky, Gilliam, and Cornfield, 1953; Wynder and Cornfield, 1953; Breslow, Hoaglin, Rasmussen, and Abrams, 1954; Watson and Conte, 1954). All these studies agree in showing that thereare more heavy smokers and fewer non-smokers among patients with lung cancer than among patients with other diseases. With one exception (the difference between the proportions of non--smokers found by McConnell, Gordon, and Jones) these differences are large enough to be important. While, therefore, the various authors have all shown that there is an" association" between lung cancer and the amount of tobacco smoked, they have differed in-their interpretation. Some have considered that the only reasonable explanation is that smoking is a factor in-the production of the disease; others have not been prepared to deduce causation and have left the associa-tion unexplained.

Further retrospective studies of that same kind would seem to us unlikely to advance our knowledge materially-or to throw any new light upon the nature of the association. If, too, there were any undetected flaw in the evidence that such studies have produced, it would be exposed only by some entirely new approach. That approach we considered should be" prospective."* It should determine the frequency with which the disease appeared, in the future, among groups of persons whose smoking habits were already known.