In this presentation from Neuro-Ophthalmology Subspecialty Day 2011, Dr. Mark Moster discusses a case of a woman with 4 months of diplopia and worsening headache. The diplopia is binocular and vertical, and began only with left head tilt before progressing to other positions. Neurologic consultation revealed a normal examination, and MRI showed a few nonspecific white matter lesions, which raised the possibility of demyelinating disease. She was referred for neuro-ophthalmologic consultation and is finally diagnosed with a trochlear nerve schwannoma. This case illustrates why the differential diagnosis of all underlying causes of fourth nerve palsy and its mimickers must be considered carefully.