Medullary thyroid carcinoma (MTC) is an uncommon thyroid tumor that usually behaves aggressively. After resection, serological surveillance for calcitonin and carcinoembryonic antigen (CEA) is used to prompt a radiographic search for metastatic disease. We report a case of a 65-year-old woman who presented with a large liver metastasis 37 years after she underwent thyroidectomy for organ-confined MTC. Her clinical course over that time showed a smoldering pattern in which she was symptom free until presentation even though her serum calcitonin and CEA concentrations were elevated for 17 years, and a small equivocal radiographic lesion in the liver was detected 10 years prior to presentation. Cytopathology from an ultrasound guided fine needle aspiration of the hepatic lesion was diagnostic for metastatic MTC. This case highlights the ability for smoldering residual MTC to suddenly transform to aggressive biological behavior after a long period of clinical remission. Diagn. Cytopathol. 2015;43:45–48. © 2014 Wiley Periodicals, Inc.