Radial and dorsal subluxation of the base of the first metacarpal is a frequent clinical and radiological phenomenon in association with the common condition of osteoarthrosis of the base of the thumb. In an attempt to determine if any ligamentous instability could be responsible for this displacement, a series of seventeen joints was dissected. The ligamentous structure is constant. Division of the first intermetacarpal ligament was alone found to be responsible for the development of the characteristic displacement of the base of the first metacarpal. In two specimens in which there was advanced osteoarthrosis, spontaneous attenuation of the first intermetacarpal ligament was demonstrated. In attempts at operative reconstruction it is the first intermetacarpal ligament which must be reinforced to provide stability.