At referral at the age of 1.5 years, the child showed an extreme difference in rotation of the head between active and passive motion toward the right side. He did not actively rotate his head more than slightly over a neutral position toward the right side but he had full active range of motion (AROM) toward the left side. Passive range of motion (PROM) was excellent in both rotation and lateral flexion bilaterally. He tilted his head toward the right side, and had stronger lateral flexor muscles on the right side. As there was no limited PROM, this indicated a right-sided postural torticollis. However, the extreme problem of actively rotating toward the right side was disconcerting. At the age of six years, hypoplasia of the sternocleidomastoid (SCM) muscle on the left side was revealed. The small amount of muscle mass on the left side explained the extremely limited active rotation toward the right side. The SCM muscle on the left side rotates the head toward the right side. Conclusion: When torticollis with good PROM is combined with unusually large problems with AROM in rotation, hypoplasia/aplasia of the SCM muscle must be investigated.