MICROSCOPIC DESCRIPTION: The locules of the main mass are lined partly by skin-like tissue with epidermis and skin adnexae, but there are areas with hollow viscus-like structures composed of various types of mature glandular epithelium, including respiratory epithelium, small bowel epithelium and cuboidal epithelium, with adjacent layers of smooth muscle. There is also mature neural tissue including nerve bundles and ganglion cells, mature adipose tissue and clustered blood vessels. The thicker parts of the cyst wall and the separate rounded fragment contain spaces lined by glandular epithelium and surrounded by smooth muscle. A few lymphoid aggregates are present. Immature elements are not seen. The adrenal gland is normal and the loose tissue is congested but otherwise unremarkable soft tissue. COMMENT: Primary retroperitoneal teratomas are rare (Gatcombe et al, J Surg Oncol 2004;86:1007-113) and metastasis from a testicular primary should be excluded clinically. Although no immature or malignant component has been identified, mature teratomas in adult males may not be completely benign. As the lesion was received in at least two pieces, completeness of excision cannot be assessed. DIAGNOSIS: Retroperitoneal tumour: Mature cystic teratoma.