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Background About 30% of men with localised prostate cancer undergoing radiotherapy with curative intent have disease recurrence associated with progression-related symptoms and substantial toxicity of salvage therapies. Previous studies with aglatimagene besadenovec (CAN-2409, hereafter referred to as aglatimagene) showed synergy with radiation and immune-mediated cytotoxicity in patients with prostate cancer. We aimed to assess whether addition of aglatimagene plus prodrug (valacyclovir) to standard-of-care external beam radiation therapy (EBRT) could improve disease-free survival in this population. Methods We conducted a phase 3, randomised, double-blind, placebo-controlled trial at 51 medical centres (26 community and 25 institutional or military) across the USA and Puerto Rico in patients with intermediate or high-risk prostate cancer. Patients aged at least 18 years who were planning to undergo EBRT and with an Eastern Cooperative Oncology Group score of 0-2 were eligible. Patients were randomly assigned (2:1) via central block-randomisation to receive either three courses of intraprostatic aglatimagene (5 × 10 11 viral particles) plus valacyclovir or placebo plus valacyclovir, with randomisation stratified by risk category and androgen deprivation therapy (ADT) use. Patients received standard-of-care EBRT (78 Gy in 2 Gy fractions) or hypofractionated EBRT (60 Gy in 3 Gy fractions or 70 Gy in 2·5 Gy fractions) with optional ADT. The primary endpoint was disease-free survival, defined as time-from-randomisation to prostate cancer recurrence or death in the intent-to-treat population (all randomly assigned patients). Safety was assessed in all individuals who received at least one injection. The trial is registered at ClinicalTrials.gov, NCT01436968, and long-term follow-up is ongoing. Findings Between Feb 21, 2012, and Sept 9, 2021, 745 men (591 [79%] White, 121 [16%] Black) were randomly assigned to receive aglatimagene plus valacyclovir (n=496) or placebo plus valacyclovir (n=249). After a median follow-up of 50·3 months (IQR 35·2-63·3), median disease-free survival was not reached (95% CI 121·78 to not reached) in the aglatimagene plus valacyclovir group versus 86·1 (IQR 29·7-143·0) months in the placebo plus valacyclovir group (hazard ratio 0·70, 95% CI 0·52-0·94; p=0·016). Treatment-emergent adverse events (TEAEs) of grade 3 or worse occurred in 40 (8%) of 479 patients in the aglatimagene group and 17 (7%)of 232 patients in the placebo group. The most common TEAE of grade 3 or worse was acute kidney injury in both the aglatimagene group (nine [2%] of 479 patients) and the placebo group (four [2%] of 232 patients). Serious adverse events occurred in 28 (6%) of 479 patients in the aglatimagene group and 17 (7%) of 232 in the placebo group. Treatment-related serious adverse events occurred in eight (2%) patients in the aglatimagene group (four acute kidney injury, two pyrexia, and one each influenza-like symptoms and urinary retention) and five (2%) in the placebo group (four acute kidney injury, and one each increased creatinine levels and skin rash; one patient reported two serious adverse events). No treatment-related deaths were reported. Interpretation Aglatimagene plus valacyclovir was associated with longer disease-free survival than placebo plus valacyclovir when added to standard of radiotherapy for the treatment of localised prostate cancer, offering a meaningful benefit without increasing clinically significant toxicity. Funding Candel Therapeutics and US National Institutes of Health.