A 44-year-old woman presented with a 1-month history of right-sided tearing, redness, and chronic headache. Clinical examination was notable for 4-mm proptosis and decreased visual acuity OD. Orbital CT demonstrated isolated enlargement of the right lateral rectus. Surgical biopsy was undertaken after an unsuccessful trial of oral steroids. Histopathology and immunophenotyping demonstrated a lymphoplasmacytic lymphoma. External beam radiation induced regression of the lymphoma with decreased proptosis and improved visual acuity.