Human T-cell leukemia virus type III (HTLV-III) was recently identified as the probable etiologic agent of the acquired immune deficiency syndrome (AIDS). Here it is shown that, in human T-cell lines infected with HTLV-III, gene expression directed by the long terminal repeat sequence of this virus is stimulated by more than two orders of magnitude compared to matched uninfected cells. The rate of transcription of the HTLV-III long terminal repeat is more than 1000 times that of the SV40 early promoter in one infected cell line. Thus, HTLV-III, like HTLV-I, HTLV-II, and the bovine leukemia virus, is characterized by trans -activation of transcription in infected cells. The efficiency of trans -activation in the case of HTLV-III may account, at least in part, for the virulent nature of HTLV-III infection.