The authors describe a case of a 27-year-old man with progressive diplopia and proptosis in whom imaging studies revealed the presence of a mass in the superior rectus muscle. Biopsy showed an infiltrative tumor of mature adipocytes with striated muscle fibres, corresponding to an intramuscular lipoma. Although extraocular muscle lipomas are very rare, with only two cases reported, they must be considered in the differential diagnosis of an orbital mass. They may cause significant morbidity to the patient, thus requiring surgical removal. Complete excision is usually difficult because of their infiltrative nature to the surrounding muscular fibres and their tendency to recur.