The most recent U.S. Census reported that Hispanics are now the nation's largest minority group. At the same time, increasing attention has focused on the inherent heterogeneity of the U.S. Hispanic population. Such a rapidly growing but heterogeneous minority poses potential challenges to population-based research. To understand those challenges better, we first considered the history of the demographers' question: "Who is Hispanic?" We then considered the implications of differing Hispanic identity criteria for disease surveillance. Although relevant to political and socioeconomic considerations, the Hispanic ethnic category may not be specifically useful for understanding most disease processes. For epidemiologic studies, there is need for more transparent criteria to classify subpopulations. Those criteria must be regularly subjected to analysis and validation.