This trial aims to assess the safety and effectiveness of using stem cells derived from donated fat for the treatment of perianal fistulas in patients with Crohn's disease. The trial is being sponsored by CELLERIX, S.A and will be conducted at several sites in Europe. The trial proposes to recruit around 200 subjects who have Crohn's disease with complex perianal fistulas. Participants will be randomly allocated into an experimental group which will receive an injection of a proprietary fat derived and expanded stem cells directly into the fistula, or into the control group that will receive a placebo injection only. Participants will be monitored for complications and for fistula healing. The trial commenced in June 2012 and is expected to be completed by 2015.