This paper deals with the role of grouting when repairing ancient masonry walls. The wall section considered is composed by two well built stone faces and a core filled with rubble. The role of grouting has been studied by means of a finite element model taking into account all the constituants of the structure. This model allows the determination of a damaged zone in which the grout is supposed to act principally. The importance of two of the mechanical properties, Young's modulus and cohesion, on the behaviour of the wall section has been analysed through a parametric study. The first results being promising, a more complete study associated with an appropriate experimental analysis could contribute to establish some relations between the properties of the materials in situ and those of the grout to be used.