An increase in the level of polycythemia rubra vera -1 (PRV-1) mRNA has been reported in some myeloproliferative disorders. We have studied the effects of PRV-1 on cell proliferation and cell survival. In cell growth assays, the number of heterologous cells expressing PRV-1 increased faster than sham-transfected cells, a difference that was more pronounced in serum-free media. Even after 5 days of exposure to serum-free media, cells expressing PRV-1 continued to proliferate, whereas the control cells ceased to proliferate. We conclude that PRV-1 is a pro-proliferation molecule, and hypothesize that its overexpression may have a role in the pathogenesis of myeloproliferative disorders.