Background Radical whole-gland therapy can lead to significant genitourinary and rectal side-effects for men with localised prostate cancer. We report on whether selective focal ablation of unifocal and multifocal cancer lesions can reduce this treatment burden. Methods Men aged 45-80 years were eligible for this prospective development study if they had low-risk to high-risk localised prostate cancer (prostate specific antigen [PSA] <= 15 ng/mL, Gleason score <= 4 + 3, stage <= T2), with no previous androgen deprivation or treatment for prostate cancer, and who could safely undergo multiparametric MRI and have a general anaesthetic. Patients received focal therapy using high-intensity focused ultrasound, delivered to all known cancer lesions, with a margin of normal tissue, identified on multiparametric MRI, template prostate-mapping biopsies, or both. Primary endpoints were adverse events (serious and otherwise) and urinary symptoms and erectile function assessed using patient questionnaires. Analyses were done on a per-protocol basis. This study is registered with ClinicalTrials.gov, number NCT00561314. Findings 42 men were recruited between June 27, 2007, and June 30, 2010; one man died from an unrelated cause (pneumonia) 3 months after treatment and was excluded from analyses. After treatment, one man was admitted to hospital for acute urinary retention, and another had stricture interventions requiring hospital admission. Nine men (22%, 95% CI 11-38) had self-resolving, mild to moderate, intermittent dysuria (median duration 5.0 days [IQR 2.5-18.5]). Urinary debris occurred in 14 men (34%, 95% CI 20-51), with a median duration of 14.5 days (IQR 6.0-16.5). Urinary tract infection was noted in seven men (17%, 95% CI 7-32). Median overall International Index of Erectile Function-15 (IIEF-15) scores were similar at baseline and at 12 months (p=0.060), as were median IIEF-15 scores for intercourse satisfaction (p=0.454), sexual desire (p=0.644), and overall satisfaction (p=0.257). Significant deteriorations between baseline and 12 months were noted for IIEF-15 erectile (p=0.042) and orgasmic function (p=0.003). Of 35 men with good baseline function, 31 (89%, 95% CI 73-97) had erections sufficient for penetration 12 months after focal therapy. Median UCLA Expanded Prostate Cancer Index Composite (EPIC) urinary incontinence scores were similar at baseline as and 12 months (p=0.045). There was an improvement in lower urinary tract symptoms, assessed by International Prostate Symptom Score (IPSS), between baseline and 12 months (p=0.026), but the IPSS-quality of life score showed no difference between baseline and 12 months (p=0.655). All 38 men with no baseline urinary incontinence were leak-free and pad-free by 9 months. All 40 men pad-free at baseline were pad-free by 3 months and maintained pad-free continence at 12 months. No significant difference was reported in median Trial Outcomes Index scores between baseline and 12 months (p=0.113) but significant improvement was shown in median Functional Assessment of Cancer Therapy (FACT)-Prostate (p=0.045) and median FACT-General scores (p=0.041). No histological evidence of cancer was identified in 30 of 39 men biopsied at 6 months (77%, 95% CI 61-89); 36 (92%, 79-98) were free of clinically significant cancer. After retreatment in four men, 39 of 41 (95%, 95% CI 83-99) had no evidence of disease on multiparametric MRI at 12 months. Interpretation Focal therapy of individual prostate cancer lesions, whether multifocal or unifocal, leads to a low rate of genitourinary side-effects and an encouraging rate of early absence of clinically significant prostate cancer.