The failure and/or toxicity of conventional therapies for many types of human cancers underscore the need for development of safe and effective alternative treatments. Toward this goal, we describe the direct oncolytic activity of RNA-based vectors derived from poliovirus, termed replicons, which are genetically incapable of producing infectious virus. These replicons are cytopathic in vitro for human tumor cells originating from brain, breast, lung, ovary, and skin (melanoma). The cytopathic effects in a malignant glioma cell line were associated with nuclear DNA condensation, indicative of cells undergoing apoptosis. Injection of replicons into established xenograft flank tumors in scid mice resulted in oncolytic activity and extended survival. Inoculation of replicons into established intracranial xenograft tumors in scid mice resulted in tumor infection within 8 h and extended survival. Histological analysis revealed that replicons had infected tumor cells at the site of inoculation and, most importantly, diffused to infect tumor cells that had metastasized from the initial site of implantation. The wide spectrum of cytopathic activity for human tumors combined with effective distribution after in vivo inoculation establishes the therapeutic potential of poliovirus replicons for a variety of cancers.