A new sign in Duchenne muscular dystrophy is described. The sign demonstrates a unique characteristic of this disorder: selective hypertrophy and wasting in different muscles of the same region. The patients were asked to abduct their arms to about 90 degrees with elbows flexed to 90 degrees and hands directed upwards. Those who could not abduct the arm to 90 degrees were asked to do so to the maximum that they could. In this posture, all patients had examination of pectoral girdles from behind. Some patients with spinal muscular atrophy and other forms of muscular dystrophies were also examined in the same manner. In this posture, patients with Duchenne muscular dystrophy demonstrated a linear or oval depression (due to wasting) of the posterior axillary fold with hypertrophied or preserved muscles on its 2 borders (i.e., infraspinatus inferomedially and deltoid superolaterally), as if there were a valley between the 2 mounts. The sign was specific to Duchenne muscular dystrophy with sensitivity of about 90%. It was most remarkable in patients 8-11 years of age.