A membrane invasion culture system was used to study the ability of EBV to enhance invasion and migration of nasopharyngeal carcinoma (NPC) cells. Semi-reverse transcriptase-PCR analysis of matrix proteinases and angiogenic factors from EBV-infected, or EBV-positive (EBV+), cells demonstrated different degrees of elevated gene expression. In our animal model, EBV+ tumors grew faster and larger than EBV-free, or EBV-negative (EBV-), tumors and also had clonal EBV terminal repeat sequences. Double-localization of EBV and certain host proteins in EBV+ tumors and biopsy specimens demonstrated that EBV up-regulates host genes only in cells that express those genes but not in cells that do not express them. Double-localization of EBV and host genes in NPC biopsy specimens all showed EBV- tumor cells expressing those host genes. Our data strongly suggest that EBV infection enhances progression of NPC tumor growth. They do not rule out a role for EBV infection in the induction and early promotion of NPC development. Unidentified factors may also enhance NPC tumor growth independent of the effects of EBV.