Abstract. During the first decades of the 20th century the lung cancer death rate increased sharply in developed countries, including Canada. An association with cigarette smoking was suspected. Between 1950 and 1960 many epidemiological studies confirmed this association. The validity of these studies was attacked fiercely by some renowned statisticians. The resulting debate sharpened the methodological and analytical rigour of epidemiological studies. Epidemiology became an indispensable tool in cancer prevention.RCsumb. Pendant les premi6res d6cennies du XXe siecle la mortalit6 attribuable au cancer du poumon augmentait nettement dans les pays industrialids, le Canada compris. On a soupconne une relation avec la fum6e de la cigarette. Entre 1950 et 1960 ungrand nombre &etudes 6pidemiologiques ont corrobor6 cette relation. Plusieurs statisticiens r&ncnnmes ont attaque fhrocement la validit6 de ces etudes. Le d6bat suivant a aiguis6 la rigeur m6thologique et analytique des etudes Cpidemioloqiues. IXpid6miologie est devenue un outil indispensable pour la prhention du cancer.In his magisterial survey of the major epidemics of the 20th century, Sir Richard Doll2 identified lung cancer and coronary thrombosis as the , two most important causes of premature death. Although both epidemics have begun to wane, lung cancer is still the leading cause of cancer mortality in Canada, accounting for about 30% of cancer deaths in males and 20% of cancer deaths in females.3 It is now acknowledged that the epidemic of lung cancer was due primarily to cigarette smoking, and some progress has been made in the campaign to reduce the preva-