The possibility that lymphoproliferative diseases may have a viral etiology has been discussed for many years. The first piece of evidence - in combination with convincing epidemiological data - was the detection of a virus of the HTLV group (HTLV I, ATLV) in patients with adult T-cell leukemia in Japan. There have been no comparable observations made in Europe. In 7 of 72 sera of patients with T-lymphoproliferative diseases in Central Europe, we were able to detect antibodies against HTLV I (ELISA and immunoblot), which might be an indication that retroviruses play a role in the etiology of these diseases.