A prospective radiological study was performed on 170 Colles' fractures which were reduced and treated in plaster. Twenty-nine fractures, which displaced and which needed a further reduction and external fixation, were excluded. The mean length of the radius decreased during plaster treatment to the same position as before the initial reduction. Radial angular compression also increased significantly during treatment. Among the seventeen fractures which were malunited, twelve had definitely displaced after the 11th day.