The human T-lymphotropic viruses type I (HTLV-I) and type II (HTLV-II) are closely related but distinct retroviruses that can infect humans. They differ from the human immunodeficiency viruses that cause the acquired immunodeficiency syndrome. Screening of the U.S. blood supply for HTLV-I/II, which began in 1988, identifies HTLV-I- and HTLV-II-infected persons who need to be counseled regarding their infections. This paper summarizes current information about the HTLV viruses and presents guidelines developed by the Centers for Disease Control and a United States Public Health Service working group for counseling persons infected with HTLV-I and HTLV-II.