BACKGROUND: Percutaneous tibial nerve stimulation (PTNS) is a new ambulatory therapy for faecal incontinence. Data from case series suggest it has beneficial outcomes in 50-80% patients; however its effectiveness against sham electrical stimulation has not been investigated. We therefore aimed to assess the short-term efficacy of PTNS against sham electrical stimulation in adults with faecal incontinence. METHODS: We did a double-blind, multicentre, pragmatic, parallel-group, randomised controlled trial (CONtrol of Faecal Incontinence using Distal NeuromodulaTion [CONFIDeNT]) in 17 specialist hospital units in the UK that had the skills to manage patients with faecal incontinence. Eligible participants aged 18 years or older with substantial faecal incontinence for whom conservative treatments (such as dietary changes and pelvic floor exercises) had not worked, were randomly assigned (1:1) to receive either PTNS (via the Urgent PC neuromodulation system) or sham stimulation (via a transcutaneous electrical nerve stimulation machine to the lateral forefoot) once per week for 12 weeks. Randomisation was done with permuted block sizes of two, four, and six, and was stratified by sex and then by centre for women. Patients and outcome assessors were both masked to treatment allocation for the 14-week duration of the trial (but investigators giving the treatment were not masked). The primary outcome was a clinical response to treatment, which we defined as a 50% or greater reduction in episodes of faecal incontinence per week. We assessed this outcome after 12 treatment sessions, using data from patients' bowel diaries. Analysis was by intention to treat, and missing data were multiply imputed. This trial is registered with the ISRCTN registry, number 88559475, and is closed to new participants. FINDINGS: Between Jan 23, 2012, and Oct 31, 2013, we randomly assigned 227 eligible patients (of 373 screened) to receive either PTNS (n=115) or sham stimulation (n=112). 12 patients withdrew from the trial: seven from the PTNS group and five from the sham group (mainly because they could not commit to receiving treatment every week). Two patients (one in each group) withdrew because of an adverse event that was unrelated to treatment (exacerbation of fibromyalgia and rectal bleeding). 39 (38%) of 103 patients with full data from bowel diaries in the PTNS group had a 50% or greater reduction in the number of episodes of faecal incontinence per week compared with 32 (31%) of 102 patients in the sham group (adjusted odds ratio 1·28, 95% CI 0·72-2·28; p=0·396). No serious adverse events related to treatment were reported in the trial. Seven mild, related adverse events were reported in each treatment group, mainly pain at the needle site (four in PTNS, three in sham). INTERPRETATION: PTNS given for 12 weeks did not confer significant clinical benefit over sham electrical stimulation in the treatment of adults with faecal incontinence. Further studies are warranted to determine its efficacy in the long term, and in patient subgroups (ie, those with urgency). FUNDING: National Institute for Health Research.