This article is concerned with an apparent similarity between the conceptions of human nature found in the early work of Jean-Paul Sartre and certain forms of transhumanism, and the role of a particular conception of human nature in the application of transhumanist ideas to debates on performance-enhancement. The article begins with a brief outline of major features of Sartre's phenomenological work (?I). The article then gives a more detailed account of the relationship between Sartre's phenomenological ontology and the view of (...) human nature expressed in Existentialism and Humanism (?II). This is followed by an outline of the central features of Sartre's later work that have a bearing on his view of human nature (?III). This allows the articulation of two possible different uses of the expression ?human nature?. A contrast is drawn between the more satisfactory of the two and the view of human nature that appears to motivate transhumanism (?IV). The article will conclude by considering the implications of Sartre's view on human nature for the advocacy of what purports to be a transhumanist project in relation to performance-enhancement in sport (?V). (shrink)