Background. Gene-directed enzyme prodrug therapy (GDEPT) based on the E. coli enzyme purine nucleoside phosphorylase (PNP) represents a new approach for treating slow growing tumours like prostate cancer (PCa). Expressed enzyme converts a systemically administered prodrug, fludarabine phosphate, to a toxic metabolite, 2-fluoroadenine. Infected and neighbouring cells are killed by a bystander effect that results from the inhibition of DNA and RNA synthesis. Methods. These studies were carried out using the transgenic adenocarcinoma of the prostate (TRAMP) model that mimics human PCa development and progression. Control TRAMP mice were injected intraprostatically with vector vehicle and thereafter intraperitoneally with saline or fludarabine phosphate (∼600 mg/m2/day) once daily for 5 consecutive days. Treated mice received a single intraprostatic injection containing 1010 particles of OAdV220, an ovine atadenovirus which expresses the E. coli PNP gene under the control of the Rous sarcoma virus promoter, followed by systemic fludarabine treatment. The weight of the genitourinary tract, seminal vesicles and the prostate as well as animal survival were monitored. Tumours were also analysed histologically. Results. Preliminary studies showed that fludarabine alone caused no significant change in genitourinary (GU) tract weight in TRAMP mice. Animals injected with vector and prodrug showed a significant reduction (36-47%) in GU tract weight (ANOVA p = 0.0002) and a 35-50% reduction in seminal vesicle weight (ANOVA p = 0.0007). In particular, the target organ showed a significant 57% reduction in prostate weight (ANOVA p = 0.0007). PNP-GDEPT mice also showed a survival advantage over control mice. Histological analysis suggested that the cancer progression was slowed in GDEPT-treated animals. Conclusion. A single course of GDEPT based on OAdV-delivered PNP and fludarabine produced highly significant suppression of PCa progression in immune-competent TRAMP mice. Copyright © 2004 John Wiley & Sons, Ltd.