A 47-year-old woman who presented with abdominal pain was found to have a 20 cm cystic retroperitoneal mass. Pathology indicated a colonic-type adenocarcinoma arising in a primary retroperitoneal mature cystic teratoma. The adenocarcinoma was predominantly intracystic with focal superficial invasion into the cyst wall but not beyond the teratoma capsule. Immunohistochemistry showed that the adenocarcinoma cells were diffusely positive for cytokeratin 20 (CK20) and caudal-type homeobox transcription factor-2 (CDX2) but negative for CK7, confirming the colonic phenotype. In addition, the adenocarcinoma was seen adjacent to teratomatous colonic-type mucosa with adenomatous change (i.e. adenoma), suggesting that it was probably arising from a colonic-type adenoma within the teratoma. The carcinoma had a higher Ki-67 proliferation index and had a higher percentage of cells stained for p53 than the adjacent adenomatous lesion. To the authors' knowledge this is the first documented case in which a colonic-type adenocarcinoma was seen arising from a precursor lesion (i.e. a colonic-type adenoma in a primary retroperitoneal mature cystic teratoma) and is the second case of intestinal-type adenocarcinoma arising in a primary retroperitoneal mature cystic teratoma.