Recent studies cast doubt on the previously held assumption that cigarettes of low smoke yield are less hazardous to the smokers' health, because of the tendency of many smokers to compensate for the lower yield by engaging in more aggressive inhalation practices and by increasing daily cigarette consumption. These studies have either looked at a static association between FTC yields and biological markers, or have asked smokers to temporarily switch to lower yield cigarettes for experimental purposes. Both of these approaches are deficient: the first, because brand switching is a dynamic process that cannot be evaluated from a single snapshop of an apparent relationship between smoke yield and biological markers. The second approach is unnatural, since the smokers switch brands solely for experimental purposes, leading to confounding factors of motivation and cigarette acceptability. The scientifically sound approach to addressing this issue is to study smokers who have voluntarily switched to lower yield cigarettes over a relatively long period of time, unencumbered by experimental stress. This is the approach we propose in this grant application. We have plasma cotinine and carboxyhemoglobin values on approximately 2,000 smokers. We propose to recontact these smokers, and test approximately 200 of them who have voluntarily switched to lower yield cigarettes. Comparisons between previous and recent biological markers will allow for a valid estimation of biological marker changes associated with a natural, voluntary switch to lower yield cigarettes.