Primary small cell undifferentiated carcinomas (SCUCs) are unusual tumors of the colon and rectum. Histologically, these lesions represent a spectrum of neuroendocrine differentiation, with oat cell carcinoma being the most primitive subtype and carcinoid tumors being the most differentiated. This observation is supported by immunohistochemical and ultrastructural findings. We report a case of SCUC of the rectum in a patient with ulcerative colitis. To date, there have been only two reported cases of primary SCUC associated with ulcerative colitis. Recent theories of tumorigenesis attribute most colorectal cancers to a single, pluripotential mucosal stem cell, regardless of the tumor's histologic type.