In explaining clinical problems, medicine aims to bring meaning to the ways in which disease and illness are perceived and manipulated. The medical explanation of AIDS illustrates three important features of this enterprise. The essay's first section discusses the ways in which contemporary medicine has successively explained AIDS in terms of a syndrome, an etiological agent, and a model. The movement through these explanatory levels reflects medicine's effort to know truly and to know effectively, i.e., to understand clinical problems in a way that will facilitate their resolution within the life of a particular sociocultural setting. The second section explores how these epistemic (knowledge-gathering) and nonepistemic (action-oriented) concerns of clinical explanation interact in the explanation of AIDS. The last section discusses the role that negotiation plays in fashioning clinical explanation by examining in greater detail the ways in which nonepistemic concerns have shaped the understanding of AIDS. 30 notes.