A 17-year-old woman developed a sixth cranial nerve palsy from a malignant peripheral nerve sheath tumor of the trigeminal nerve. This case is unusual in that the principal symptom was diplopia stemming from a sixth cranial nerve palsy. Pain was mild, and trigeminal function was preserved. Imaging evidence of rapid growth of the cavernous sinus mass gave rise to an initial impression that the cause might be inflammatory. Treatment with gamma knife stereotactic radiosurgery produced some improvement in sixth cranial nerve function and reduction in tumor size over a follow-up period of 9 months.