Despite the ubiquity of gasoline for several decades and more recent modifications in fuel formulations to achieve “cleaner” gasoline, a quantitative comparative assessment of the health risks related to these fuels remains to be performed. Under authority of Clean Air Act section 211(b), EPA has required manufacturers of certain oxygenates, including methyl tertiary butyl ether (MTBE) and other ethers and alcohols, to conduct toxicity studies on mixtures of individual oxygenates in gasoline, as well as baseline gasoline alone. Because more extensive testing (e.g., 2-year inhalation bioassays) was required for baseline gasoline and MTBE-gasoline than for the other oxyfuels, a direct quantitative comparison of the health effects of these fuels should be possible. In addition, studies comparing personal exposure levels to fuels and fuel by-products in cities using either non-oxygenated or oxygenated gasoline were required under CAA 211(b). Based on these and other data, a comparative risk characterization is planned for these fuels because of their national and international significance.