Prostate cancer is the leading internal malignancy and the second most common cause of cancer deaths in American men. Current curative therapies for prostate cancer include surgery and external radiation therapy; however, both have significant morbidity and their impact on disease control has been less than complete, especially with radiotherapy. A novel approach to the control of cancer growth is gene therapy, a field that has progressed rapidly. Direct introduction of therapeutic genes into malignant cells in vivo may provide an effective treatment of solid tumors, including tumors of the prostate. One system employs the herpes simplex virus thymidine kinase (HSV-tk) gene, which codes for an enzyme that phosphorylates the nucleoside analogue ganciclovir (GCV) into a phosphorylated intermediate that is incorporated into newly synthesized DNA and terminates further replication, leading to cell death. Since normal mammalian cells do not possess this enzyme, cytotoxicity depends on the success successful introduction and expression of the HSV-tk gene, phosphorylation of ganciclovir, and synthesis of DNA. Using both human and animal models for prostate cancer, we have demonstrated that adenovirus- mediated transfer of the HSV-tk gene and GCV treatment resulted in ablation of the tumors. This Phase I study is designed to determine the safety and efficacy of gene therapy for patients with recurrence of prostate cancer after radiation therapy. Patients with locally recurrent prostate cancer after definitive radiation therapy do not have any standard treatment available that has been demonstrated to have a high degree of efficacy in eradicating the tumor with a reasonable degree of safety. Thus, the potential risks associated with the use of gene therapy in this group would appear reasonable. Tumors will be treated with transrectal ultrasound-guided intratumor injections of replication-defective adenovirus vector delivering the HSV-tk gene. Following virus injection, patients will be hospitalized for 2 weeks at which time they will receive twice-daily intravenous infusion of ganciclovir. Only one course of therapy will be administered. Each patient will be carefully monitored for cytopathic or toxic effects. Five patients will be tested with a low dose of virus and if there are no serious adverse side effects, the dose will be escalated in two subsequent groups of 5 patients, or until unacceptable toxicity is reached. Effectiveness will be monitored by serum prostate-specific antigen (PSA), transrectal ultrasound of the prostate, prostate biopsy, and comparison of survival times to historical survival times for patients with radiation recurrent prostate tumors. The primary objective of this initial study is to determine whether the treatment is associated with significant toxicity.