In the 10 year period since its discovery, human T cell leukemia virus type II (HTLV-II) has emerged as a significant human pathogen. Infection has been shown to be endemic in a large number of native Indian populations throughout the Americas, and high rates of infection have also been demonstrated in intravenous drug abusers (IVDAs) in both the United States and Europe. While the role of the virus in human disease remains to be clearly defined, there is accumulating evidence that it may be associated with neurological, and perhaps, rare lymphoproliferative disorders. This review summarizes the current status of our understanding of HTLV-II infections, and will emphasize important aspects of the epidemiology of infection, and the molecular and biological properties of the virus. In addition, the salient features of the clinical disorders known to be associated with infection are described. © 1994 Academic Press. All rights reserved.