To report a case of carcinoma ex pleomorphic adenoma observed during the patient's first operation.

A 63-year-old female presented with proptosis and ptosis that was aggravated 1 year prior. On preoperative CT image, a 32 × 20 × 21 mm-sized well demarcated mass (suspected as pleomorphic adenoma) was observed and was removed entirely by anterolateral orbitotomy. The excised mass surface was uneven but the capsule appeared intact on gross examination. Hard, yellow-colored and soft, dark-colored materials were found concurrently on cross section. The histological examination showed malignant cells as part of the soft material and was diagnosed as carcinoma ex pleomorphic adenoma.

We report a case of carcinoma ex pleomorphic adenoma of the lacrimal gland that presented with malignant change during the patient's first operation. Supposedly, during the process of mass growth, minimal rupture occurred causing malignant transformation. Clinically, although a mass is believed benign based on imaging, the possibility of malignant transformation of a tumor increasing rapidly or enlargement causing development of rapid proptosis should be considered.