A medical risk factor is a condition that increases the likelihood that a person will contract a disease. The term pervades current writing and discussion about health, with physicians incorporating the idea in their daily practice, members of the public altering behaviors in way intended to reduce risk, insurers setting rates in accordance with their assessments of risk, and Congress enacting laws and spending funds to alter public exposure to risk factors. In short, the idea of a risk factor has become a real influence in current thinking about health and social behavior. Yet we know little about the origins of this concept, which was formed at the intersection of probability and statistics, public health, and the insurance industry. We also know little about how it became accepted, then relied upon, by various members of society. To address these gaps in our knowledge, this project will examine the history and current status of the medical risk factor, with specific attention to its application in coronary heart disease, using as source material life insurance archives, specialized journals, popular publications, and other secondary sources.