The investigators will start a national multi-centre double-blind randomized sham-controlled trial in April 2017 at 6 hospitals in Belgium and 2 in The Netherlands. They will randomly allocate 108 patients with active ulcerative colitis (Mayo score 4-10, endoscopic Mayo score 2 or 3) in a 1:1 ratio, using a pre-established randomization list, to either 'superdonor' faecal microbiota transplantation or autologous fecal microbiota transplantation (=sham). Each patient will receive 4 FMT's. At baseline FMT will be performed during sigmoidoscopy. At week 1, 2 and 3, the FMT will be administered through rectal instillation. Each FMT will be derived from one donor. Donors will be pre-selected based on a species richness and abundance of taxa of interest. The primary outcome will be steroid-free clinical and endoscopic remission at week 8 (Mayo score ≤2, all subscores ≤ 1, and ≥1 point reduction in endoscopy subscore). Fecal, blood and mucosal samples and questionnaires will be collected at different time points. 16S rRNA stool analysis will be performed to assess the microbial changes after FMT.