An investigation was done of the evidence for transmission of human immunodeficiency virus (HIV) from an HIV-positive man to several male and female sex contacts. Phylogenetic analysis of sequences from the gag and env genes showed a close relationship between the predominant virus strains from the source and 2 contacts. However, the likelihood that a female contact was infected by the source could not be determined, despite contact tracing indicating that this may have occurred. One male, shown by contact tracing and molecular evidence to have been infected by the source, subsequently transmitted HIV to his female sex partner. HIV sequence from a plasma sample used as a control in the phylogenetic analysis contained env and gag sequences that were closely related to those from the source. An epidemiologic link between these 2 individuals was subsequently confirmed by contact tracing.