WHEN smokers are asked what the tar and nicotine ratings of their cigarettes mean, they usually answer, "You know, it's what's in the cigarette." Even though much epidemiologic research and many health recommendations1,2 have become dependent on the published tar and nicotine yields (to say nothing of the dependence of cigarette advertising on these figures), many researchers and health professionals appear to have no better understanding of these ratings than does the average smoker. Cigarettes do not deliver measured doses of tar and nicotine the way a capsule delivers a measured dose of a drug. A cigarette that has only a few small puffs taken on it produces small amounts of tar. Tar and nicotine delivery are highly correlated; for the sake of brevity, only tar yields will be discussed. Tar values were analyzed to the first decimal place but are reported here with no decimal places, as they