Oncolytic viruses are an innovative therapeutic strategy for cancer, wherein viral replication and cytotoxicity are selective for tumor cells. Here we show the efficacy of systemically administered oncolytic viruses for the treatment of spontaneously arising tumors, specifically the use of oncolytic herpes simplex viruses (HSV) administered i.v. to treat spontaneously developing primary and metastatic prostate cancer in the transgenic TRAMP mouse, which recapitulates human prostate cancer progression. Four administrations of systemically delivered NV1023 virus, an HSV-1/HSV-2 oncolytic recombinant, to TRAMP mice at 12 or 18 weeks of age (presence of prostate adenocarcinoma or metastatic disease, respectively) inhibited primary tumor growth and metastases to lymph nodes. Expression of interleukin 12 (IL-12) from NV1042 virus, a derivative of NV1023, was additionally effective, significantly reducing the frequency of development of prostate cancer and lung metastases, even when the mice were treated after the onset of metastasis at 18 weeks of age. NV1042-infected cells, as detected by 5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside staining for Lac Z expressed by the virus, were present in prostate tumors 1 week after the final virus injection and viral DNA was detected at 2 weeks after final virus injection by real-time PCR in primary and metastatic tumors but not in liver or blood. No toxicity was observed in any of the treated mice. The efficacy of the IL-12-expressing NV1042 virus in this aggressive prostate cancer model using a clinically relevant treatment paradigm merits its consideration for clinical studies.