Fever of unknown origin (FUO) was originally defined as a body temperature greater than 38.3°C on several occasions longer than 3 weeks, with a diagnosis that remains unclear after 7 days of obligatory investigation. Only a few types of solid tumors have been associated with FUO. We described 2 patients who had recurrent fever but no other specific gastrointestinal symptoms where carcinoma of the colon was the only identifiable cause. In the first case, a mass arising from the sigmoid colon was found without any nodal metastasis, and the fever was resolved after three days of the surgical resection. In the second case, advanced adenocarcinoma was found in the ascending colon together with liver cirrhosis. Although it was not possible to surgically remove this tumor, prolonged fever in the patient was most likely due to the carcinoma. These cases indicate that clinicians should consider carcinoma of the colon in the differential diagnosis of patients with FUO.