Internal rotation deformity may contribute to the onset of tardy ulnar nerve palsy after a supracondylar fracture. We measured the angle of deformity in five patients with nerve palsy using an electrogoniometer and compared it with that of patients without palsy. Varus deformity of patients with nerve palsy was 23 degrees +/- 15.2 degrees (mean +/- SD), and that of patients without palsy was 12 degrees +/- 10.9 degrees. No significant difference was seen between the two groups. Internal rotation deformity of patients with nerve palsy was 16 degrees +/- 4.2 degrees, and that of patients without palsy was 2.5 degrees +/- 6.3 degrees (p < 0.05). The results suggest that internal rotation deformity contributes to the development of tardy ulnar nerve palsy.