Fifty patients with various hyperplastic and malignant lymphoproliferative diseases were investigated for evidence of human herpesvirus-6 infection. Virus DNA and antigen expression was investigated in lymph node biopsies by in situ hybridization and immunohistology and was correlated with data of immunophenotyping. Supplemental immunoglobulin- and T cell receptor gene rearrangement studies were used to support the classification of the proliferative lymphoid lesion. Elevated numbers of cells carrying HHV-6 DNA and/or antigens were found in cases of Hodgkin's lymphoma and follicular center cell non-Hodgkin's lymphoma as well as atypical polyclonal lymphoproliferation (APL), yet not in reactive lymphoid hyperplasia and in most other lymphomas. Immunophenotyping showed that virus -infected cells were primarily lympho-histiocytic elements, less frequently Hodgkin's- and Reed-Sternberg cells, and not malignant B lymphocytes as in follicular center cell lymphomas. This suggests that the virus is rather not the causative oncogen in these cases, yet does not exclude a cocarcinogenic effect of it during the development and ths course of uncontrolled lymphoproliferation.