Although retroviruses have long been associated with a variety of animal diseases, active research in the field of human retroviruses dates from the discovery of human immunodefici­ ency virus (HIV) in association with acquired immunodefici­ ency syndrome (AIDS). The enormous research efforts in this field have been directed toward understanding the nature of the virus and toward its elimination through preventive vaccin­ ation and the cure of the disease. Human T-cell leukemia virus (HTLV-l) was the first member of the human retrovirus family to be discovered. It was implicated as the cause of adult T-cell leukemia (ATL) even before the association of HIV and AIDS was established. Research on HTL V -1 has, however, been lagging behind that of HIV because of the importance of AIDS. Today HTLV-1 and possibly closely related HTLV-2 are associated with a variety of human neurologic diseases, and research activities in this field may show that human retroviruses can cause a variety of human diseases in addition to those affecting the nervous system. Papers in this volume attempt to acquaint the reader with the present state of research into retrovirus infection and related diseases of the nervous system.