Fistula in ano is a frequent disease, which often has a difficult diagnosis and mapping, and comports significant morbidity. Today, Magnetic Resonance (MRI) allows a rapid, simple and precise mapping of peri anal fistula, which leads to a correct surgical treatment. In this paper, the authors review its pathology and MRI role in diagnosis and mapping. The authors also report a retrospective and descriptive study on the MRI results for the mapping of fistula in ano performed in Hospital Fernando Fonseca for a 38-month period.