A gauge for the medullary cavity comprises a tubular rod (1) shaped as a medullary pin, having incised lines (3, 3') and one or mor grooves (3, 3') in the vicinity of its distal end (2) and a scale (4) for measuring lengths and a substantially radial handle (5) which can be attached to the tubular rod (1) along at least part of its length, in the vicinity of its proximal end. The handle (5) comprises a tubular body (6), to one end of which is fixed a sleeve (7) which is slidably mounted on the rod. The end (8) of sleeve (7) pointing towards the distal end of the rod is of frustoconical shape which can be inserted into the medullary cavity in order to check its internal diameter and its opposite end has a circumferential edge (9) which can be aligned with reference marks on the scale (4) to indicate the length of the medullary cavity. Tubular member (6) has adjustable friction means (12, 13) within it to immobilise the handle in a given longitudinal position along the rod. The gauge can check both the diameter and the length of the cavity.