The term "race" has many definitions, ranging from a family unit to a species, but in common and medical usage, defining "race" has meant separating Homo sapiens into three to six groups. This division of Homo sapiens into race taxons started in the 18th century, when the sciences of genetics and evolutionary biology were not yet invented. These disciplines have since shown that human race taxonomy has no scientific basis. Race categories are social constructs, that is, concepts created from prevailing social perceptions without scientific evidence. Despite modern proof that race is arbitrary biological fiction, racial taxons are still used widely in medical teaching, practice, and research. Human diversity is inconsistently taught in medical schools and erratically presented in medical texts. Race taxons have been "medicalized"; that is, race groupings have been legitimized by their use in medical literature and practice as acceptable descriptive labels that are integral to the proper diagnosis and treatment of disease in humans. Assumptions about disease that are made because a race has been assigned can result in important negative consequences for individual patients and inaccurate genetic inferences for populations. In contrast, ethnicity is a concept that incorporates social, religious, linguistic, dietary, and other variables to identify individual persons and populations. Ethnicity may be able to impart clinical clues to diagnosis if the clinician taking the history is well informed and open minded. Ethnic boundaries are dynamic and imprecise, and a strict methodical approach to ethnicity that is equal to the approach required for the study of other variables is necessary if the concept of ethnicity is to be clinically useful.