BACKGROUND With the differential diagnosis of a diarrhea a lot of causes has to be considered. In very rare cases diarrhea can be the first symptom of a medullary thyroid carcinoma (MTC). METHODS A 28-year-old patient came to the admission department because of persisting diarrhea. A computerized tomography revealed multiple hepatic and pulmonary metastases. A medullary thyreoid carcinoma was found as the cause of it. The serum calcitonin values were highly increased, later the carcinoembryonal antigen (CEA), too. Sandostatin, a radioimmune therapy (131J-anti-CEA antibody) and adriamycin were therapeutically applied. The patient died 24 months after the occurrence of the first symptoms. CONCLUSIONS In case of persisting diarrhea the differential diagnosis of a medullary thyroid carcinoma must be taken into consideration and a calcitonin determination has to be arranged. Yet, typical symptoms like struma nodosa, swollen neck lymph nodes or a CEA increase can still be missing in the initial phase.