Epstein-Barr virus (EBV) is closely associated with the generation of various tumors, including Burkitt's lymphoma. Human resting B cells from peripheral blood are easily transformed by EBV to actively proliferating B-lymphoblastoid cell lines (LCLs). These LCLs with normal diploid karyotypes have been believed to be "immortal", without becoming tumorigenic. A series of recent studies, however, indicate that this initial, simple concept needs extensive reconsideration. Most LCLs from normal individuals are mortal because their telomeres shorten. Some LCLs are truly immortalized by developing strong telomerase activity and aneuploidy, accompanied by various other changes: down-regulation of p16/Rb; mutation of the p53 gene; modulation of apoptosis; and sensitivity to various chemical agents. Some post-immortal LCLs additionally develop the ability to form colonies in agarose and even become tumorigenic by developing the ability to grow in nude mice. The genetic background of LCLs markedly affects the frequency of immortalization. In summary, changes of B cells after infection by EBV are roughly divided into two steps: (a) transformation of B cells into LCLs caused by EBV proteins; and (b) immortalization and tumorigenesis of LCLs mainly regulated by the factors of host cells in cooperation with EBV proteins. The new concept as reviewed here is essential for the future study of tumorigenesis by EBV.