Cubitus varus following supracondylar fracture of the humerus in children consists of varus, hyperextension, and internal rotation deformities of the distal bone fragment of the humerus. There has been no simple clinical method of measuring internal rotation deformity. The authors report here on a method of accurately measuring the internal rotation of the shoulder in a position with the elbow at 90 degrees flexion on the back and the shoulder held at the maximum extension. When a patient with developing cubitus varus attempts to rotate his shoulder internally, there is an apparent increase in the degree of internal rotation at the shoulder of the fractured side. The mean value of the angle formed between the horizontal plane of the back and the mid-line of the forearm was approximately 0 degrees in 217 normal children, whereas an increase of 35 degrees was seen in seven children with cubitus varus. When this angle was corrected successfully, the angle after surgical correction became approximately 0 degrees. In the treatment of cubitus varus deformity, it is necessary to simultaneously correct three-dimensional deformities.