A 55-year-old healthy Thai man presented with incidental finding of a well-circumscribed, 5.8×5.4 cm mass in the right side of the pelvic cavity with heterogeneous density by a CT scan performed for trauma. No other distant lesion was detected. The pathology from pelvic mass showed poorly differentiated neuroendocrine carcinoma (NEC). A submucosal mass of 10 cm from the anal verge was found at colonoscopy, and a biopsy showed the same histopathology as in the pelvic mass. Final diagnosis was poorly differentiated primary rectal NEC with metastasis to the right iliac node (by CT scan). Carboplatin and etoposide were given for six cycles and tumour size was decreased.