The incidence of prostate cancer has dramatically increased worldwide in the past decade, with mortality rates also increasing in many countries. Once prostate cancer is diagnosed, it is important to rapidly begin a treatment regimen that is either potentially curative or impedes disease progression. When the disease is confined to the prostate, it can be cured by radical prostatectomy or irradiation therapy. However, there are no curative therapies for locally advanced, recurrent, or metastatic diseases. Clearly, new therapies are needed for these patients. Gene therapy may provide additional therapeutic options with the potential to affect both localized and metastatic disease. Virus-mediated transduction of the herpes simplex virus thymidine kinase (HSV-tk) gene transfer, followed by a course of the prodrug ganciclovir (GCV), so-called suicide gene therapy, has been demonstrated by several investigators. The present in situ gene therapy clinical trial for human prostate cancer demonstrated safety, clinical efficacy, and biological effects of antitumor activity. HSV-tk clinical trials for prostate cancer are also ongoing in Japan, the Netherlands, and Mexico. Currently, numerous preclinical studies have reported immunomodulatory cytokine gene therapy, such as interleukin-2, interleukin-12, B7-1 (CD80), B7-2 (CD86) and granulocyte-macrophage colony-stimulating factor. Several clinical studies have been approved that potentially will show that these immunomodulatory gene therapies may generate an effective local and systemic antitumor activity and that should provide options for patients with prostate cancer. We review the multiple issues involved in current in situ gene therapy (gene/immunotherapy), its outcome, and future directions for patients with prostate cancer.