Although adenocarcinoma represents the vast majority of neoplasms of the large intestine, small cell undifferentiated carcinoma (SCUC) also arises from the colorectum. SCUC of the colorectum is highly malignant and shares the similarities in histologic characteristics, behavior, and histochemistry with SCUC of the lung. We report herein a case of SCUC in the ascending colon with rapid enlargement after resection. A 70-year-old male, who presented to a nearby physician with chief complaints of pain in the right lower quadrant, was referred to our hospital. We found a tumor mass that was approximately 15 cm in size in the right lower quadrant. Computed tomography (CT) images showed an irregularly shaped tumor, located inferior to the lower border of the right kidney and in the area of the ascending colon. By colonoscopy, we found a circumferential tumor of the ascending colon. A biopsy indicated it to be SCUC. The patient underwent right hemicolectomy. Two weeks after the resection, we palpated a tumor mass in the same area. The abdominal CT images showed a tumor mass that was approximately 10 cm in size. The tumor rapidly enlarged, and the patient died of multiple organ failure. SCUC is a tumor with a high malignant potential. Radical treatment cannot be achieved by surgical therapy alone and hence further studies of effective adjuvant therapy would be required.