A case of isolated unilateral temporalis muscle hypertrophy of unknown cause is presented. It had been treated surgically 10 years previously. The histologic workup showed normal muscle tissue. A year before, the patient had reported a slowly enlarging mass in the right temporal region next to the eye. The recurrence of temporalis muscle hypertrophy was idiopathic. It was managed using a combination of surgical treatment and botulinum toxin A injection.