A 65-year-old man had painless progressive proptosis of five years' duration in his left eye. Sudden increase of the proptosis required a lateral canthotomy. A lateral orbitotomy was performed and a large, encapsulated, globoid mass was totally excised. Histopathologic diagnosis was adenoid cystic carcinoma arising in and surrounding a benign mixed tumor of the lacrimal gland. The computerized tomographic features correlated quite accurately with the gross appearance and histopathologic findings observed in the lacrimal gland mass. The authors discuss the clinical and radiographic features that should alert the clinician to the possibility of malignant transformation in a benign mixed tumor of the lacrimal gland.