The human immunodeficiency viruses types 1 and 2 have been implicated as the etiologic agent of the acquired immunodeficiency syndrome and its related disorders. The direct detection of human immunodeficiency virus is complicated by the low incidence of free circulating virus as well as the small number of infected cells. An in vitro DNA amplification procedure known as the polymerase chain reaction has been applied to the detection of the human immunodeficiency virus proviral sequences in infected individuals. This article highlights the features of the polymerase chain reaction and its contribution to the detection of these viruses.