Progressive ptosis and headache developed in a 50-year-old woman with non-small cell lung cancer. Although brain magnetic resonance imaging showed improved cerebellar metastasis after prior radiotherapy without any other abnormality, the follow-up examination taken 6 months later revealed metastasis to the cavernous sinus. The diagnosis of metastasis to the cavernous sinus is often difficult because it is a very rare manifestation of lung cancer, and symptoms can occur prior to developing a radiologically detectable lesion. Therefore, when a strong clinical suspicion of cavernous sinus metastasis exists, thorough neurologic examination and serial brain imaging should be followed up to avoid overlooking the lesion.