Vascular endothelial growth factor (VEGF) is one of the main angiogenic cytokines in human solid tumours and inhibition of VEGF-induced angiogenesis suppresses tumour growth. Some groups of malignant lymphoma, including peripheral T-cell lymphomas and Hodgkin's disease, are characterized by a conspicuous proliferation of small vessels. To test the hypothesis that VEGF may also be involved in the angiogenesis in lymphomas and other lesions of the lymphoid system, VEGF expression was analysed in tissues, employing in situ hybridization with a 35S-labelled RNA probe specific for this cytokine. Significant expression of VEGF transcripts was observed in Hodgkin's disease and peripheral T-cell lymphomas, particularly of the angioimmunoblastic type. In contrast, expression of this cytokine was minimal or absent in follicle centre lymphoma and chronic lymphocytic leukemia of B-cell type. VEGF was mainly observed in reactive non-lymphoid CD68-negative cells, which probably represent fibroblasts or myofibroblasts. In normal and ulcerated tonsils, VEGF was expressed in the squamous epithelium but only rarely found in the lymphoid tissue. Although infectious mononucleosis tonsils contained high numbers of VEGF-positive cells in the interfollicular zone, expression of this cytokine was not found in Epstein-Barr virus (EBV)-infected cells, as determined by simultaneous in situ hybridization for VEGF and EBV-encoded small nuclear RNAs (EBER). In 5/8 cases of Castleman's disease, germinal centres containing small vessels also showed expression of VEGF, in contrast to normal tonsillar germinal centres which are devoid of both vessels and VEGF transcripts. It is concluded that VEGF may be involved in the induction of the angiogenesis of both peripheral T-cell lymphomas and Hodgkin's disease, but not in low-grade B-cell lymphomas. In contradistinction to solid tumours, in which this cytokine is commonly secreted by the tumour cells themselves, in malignant lymphoma VEGF is not a product of neoplastic cells. Vascularization of germinal centres in Castleman's disease may also be a consequence of abnormal local expression of VEGF.