We investigated the levels of tumor necrosis factor-beta (TNF-beta) mRNA in the tumorous tissues of a series of 18 patients with peripheral T-cell lymphomas (PTCL), to assess the contribution of the expression of this gene to the features of the disease. Total RNA, extracted from diagnostic tissue specimens, was subjected to semiquantitative analysis by reverse transcription-coupled polymerase chain reaction (RT-PCR). The level of TNF-beta mRNA was semiquantified against that in MT-2 cells, a line of human T cells infected with human T cell leukemia virus type I (HTLV-I). Expression of TNF-beta in neoplastic T-cells was confirmed by immunohistochemistry. The extent of TNF-beta gene expression was correlated with the histopathological features of neovascularization. There was also a relationship between the extent of TNF-beta gene expression and the presence of B-symptoms. Results suggest that TNF-beta produced by neoplastic T-cells influences clinical features and is involved in histopathogenesis of PTCL.