Primary paranasal sinus lymphoma is an uncommon malignancy that accounts for only 0.17% of all lymphomas and is often difficult to diagnose. Initially, tumors in this area cause very few symptoms and are at an advanced stage at the time of diagnosis. Ophthalmologic signs and symptoms occur early in the disease process because of close proximity of the orbit to the paranasal sinuses. We report on a case of primary malignant lymphoma of the sphenoid and ethmoid sinuses with involvement of the cavernous sinus, initially presenting with ophthalmologic signs and symptoms in association with cavernous sinus syndrome.