THE BIOCHEMICAL measurement of serum cotinine, the primary metabolite of nicotine, is widely applied as a marker of both tobacco use and exposure to environmental tobacco smoke (ETS). Previous studies1-4 have suggested that non-Hispanic blacks have higher levels of serum cotinine than do non-Hispanic whites who report similar levels of cigarette smoking. The interpretation of the results in these studies has been subject to debate, however.3,5-8 Some researchers3,6,7 have suggested that differences between levels of serum cotinine in non-Hispanic black smokers and non-Hispanic white smokers are attributable, at least in part, to racial differences in nicotine metabolism or elimination. Others5 believe that such differences are attributable to other variables, including differences in the type of cigarette smoked (length of cigarette, menthol or nonmenthol, filter or nonfilter, and nicotine yield) and differences in how the cigarettes are smoked (blocking ventilation holes by fingers or lips, frequency and depth of inhalation, retention time of smoke in the lungs, and percentage of available tobacco smoked). Serum cotinine differences by race have also been attributed to differences in the accuracy of cigarette smoking self-reports2,9-12 and to differences in exposure to ETS.4,13,14