To determine the safety and evaluate the efficacy of repeated administration of virus-producing cells (GLI 328) containing the herpes simplex virus thymidine-kinase gene followed by ganciclovir treatment in adults with recurrent glioblastoma multiforme, we conducted a phase I/II multi-institutional trial. Eligible patients underwent surgical resection of tumor, followed by injections of vector producing cells (VPC) into the brain adjacent to the cavity. An Ommaya reservoir placed after surgery was used to inject a further dose of VPC seven days after surgery, followed seven days later by ganciclovir. Further gene therapy was given at 28-day intervals for up to a total of five cycles. Toxicity and anti-tumor effect were assessed. Of 30 patients who enrolled in the study, 16 experienced serious adverse events possibly related to the experimental therapy. Laboratory testing, including polymerase chain reaction analysis to detect replication-competent retrovirus in peripheral blood lymphocytes and tissues, as well as co-cultivation bioassays, were negative. Before receiving ganciclovir, 37% of the patients showed evidence of transduced peripheral blood leukocytes, but only 12% showed a persistence of transduced cells at the end of the first cycle of ganciclovir. Median survival was 8.4 months. Twenty percent of the patients (n = 6) survived more than 12 months from the date of study entry. This treatment modality is feasible and appears to have some evidence of efficacy. Toxicity may be related in part to the method of gene delivery.