Starting in the 1950s, physicians and researchers began to debate the exact nature of the relationship among blood cholesterol, diet, and cardiovascular risk. Using professional medical, public health, and scientific journals, this article examines the history of a series of intense and sustained debates regarding the credibility of the diet-heart hypothesis, which proposed that diet was causally linked to coronary artery disease. Brought about by intellectual disagreements and illuminated by personal quarrels, these debates created a profound professional rift among researchers who debated whether observational data could be used to prove that dietary intake caused heart disease and who sought to differentiate between “good” and “bad” science. But while the debate persisted into the early 1980s, Americans had begun to adopt the diet-heart hypothesis as public health truth as early as the 1960s, embracing cookbooks promoting “heart healthy” diets that promised to prevent coronary artery disease. Although critics and advocates of diet-heart continued to debate the theory's finer points, the widespread adoption of diet-heart in American homes meant that the debate had become almost moot by the time the National Heart, Lung, and Blood Institute officially endorsed the hypothesis in the 1980s.