The carcinogen known to cause the most cases of cancer is what--tobacco?

Yes: According to the nci: cigarette smoking and exposure to tobacco smoke cause more than 440, 000 premature deaths each year in the United States. Of these premature deaths, about 40 percent are from cancer, 35 percent are from heart disease and stroke, and 25 percent are from lung disease. Smoking is the leading cause of premature, preventable death in this country.

Sort of: A carcinogen is a single chemical or physical agent and the identity of the agent causing the largest number of cancers is not clear. Tobacco is single material responsible for being the most preventable cause of cancer. Other common agents are alcohol, radiation, hormones, and some viruses.