Ptosis on its own is an infrequent initial manifestation of orbital lymphoma. Orbital lymphoma usually presents as a palpable mass with proptosis, diplopia, and conjunctival ("salmon-pink") swelling. We report here a 62-year-old female patient who presented with right eye ptosis. The initial imaging study showed an indistinct enlargement of the superior rectus-levator muscle complex. After 3-4 months, ptosis and upward gazing movement were further restricted. The imaging study revealed a definite soft-tissue mass in the superior orbit surrounding the superior rectus-levator muscle complex. A tumor biopsy through anterior orbitotomy revealed a large diffuse B-cell lymphoma. With the experience of this case, we suggest that orbital lymphoma should be included in the differential diagnosis of ptosis accompanied by impairment of levator muscle function.