A human retrovirus etiologically related to a unique leukemia, adult T‐cell leukemia, has been discovered. This retrovirus is endemic in certain areas and ethnic groups, such as the Japanese in Japan and Blacks in Africa. The virus is transmitted from mother to child and husband to wife and by blood transfusion. In addition, a virus like the human retrovirus has been found in various species of monkeys in Asia and Africa. This review describes and discusses recent results on the natural (...) history of these primate lymphotropic retroviruses. (shrink)