Background Vascular-targeted photodynamic therapy, a novel tissue-preserving treatment for low-risk prostate cancer, has shown favourable safety and effi cacy results in single-arm phase 1 and 2 studies. We compared this treatment with the standard of care, active surveillance, in men with low-risk prostate cancer in a phase 3 trial. Methods This randomised controlled trial was done in 47 European university centres and community hospitals. Men with low-risk, localised prostate cancer (Gleason pattern 3) who had received no previous treatment were randomly assigned (1: 1) to vascular-targeted photodynamic therapy (4 mg/kg padeliporfi n intravenously over 10 min and optical fi bres inserted into the prostate to cover the desired treatment zone and subsequent activation by laser light 753 nm with a fi xed power of 150 mW/cm for 22 min 15 s) or active surveillance. Randomisation was done by a web-based allocation system stratifi ed by centre with balanced blocks of two or four patients. Best practice for active surveillance at the time of study design was followed (ie, biopsy at 12-month intervals and prostate-specifi c antigen measurement and digital rectal examination at 3-month intervals). The co-primary endpoints were treatment failure (histological progression of cancer from low to moderate or high risk or death during 24 months' follow-up) and absence of defi nite cancer (absence of any histology result defi nitely positive for cancer at month 24). Analysis was by intention to treat. Treatment was open-label, but investigators assessing primary effi cacy outcomes were masked to treatment allocation. This trial is registered with ClinicalTrials. gov, number NCT01310894. Findings Between March 8, 2011, and April 30, 2013, we randomly assigned 206 patients to vascular-targeted photodynamic therapy and 207 patients to active surveillance. Median follow-up was 24 months (IQR 24-25). The proportion of participants who had disease progression at month 24 was 58 (28%) of 206 in the vascular-targeted photodynamic therapy group compared with 120 (58%) of 207 in the active surveillance group (adjusted hazard ratio 0 +/- 34, 95% CI 0 +/- 24-0 +/- 46; p< 0.0001). 101 (49%) men in the vascular-targeted photodynamic therapy group had a negative prostate biopsy result at 24 months post treatment compared with 28 (14%) men in the active surveillance group (adjusted risk ratio 3 +/- 67, 95% CI 2 +/- 53-5 +/- 33; p< 0.0001). Vascular-targeted photodynamic therapy was well tolerated. The most common grade 3-4 adverse events were prostatitis (three [2%] in the vascular-targeted photodynamic therapy group vs one [< 1%] in the active surveillance group), acute urinary retention (three [2%] vs one [< 1%]) and erectile dysfunction (two [1%] vs three [1%]). The most common serious adverse event in the vasculartargeted photodynamic therapy group was retention of urine (15 patients; severe in three); this event resolved within 2 months in all patients. The most common serious adverse event in the active surveillance group was myocardial infarction (three patients). Interpretation Padeliporfi n vascular-targeted photodynamic therapy is a safe, eff ective treatment for low-risk, localised prostate cancer. This treatment might allow more men to consider a tissue-preserving approach and defer or avoid radical therapy.