Colonic inflammation seen in inflammatory bowel disease (IBD) predisposes to the development of colorectal adenocarcinoma. In contrast, colorectal neuroendocrine carcinomas (NECs) have rarely been reported in the setting of IBD, and no definitive relationship between these tumours and IBD has been established. Dysplasia from chronic inflammation leading to neuroendocrine cell differentiation may be responsible for NEC development, though this finding has not been seen consistently. We present a case of large-cell neuroendocrine carcinoma of the sigmoid colon in a 65-year-old woman with long-standing ulcerative colitis. Although she underwent regular endoscopic follow-ups and was receiving the tumour necrosis factor alpha inhibitor infliximab, her tumour was large and aggressive, with metastases to the liver discovered at time of diagnosis. This case highlights the aggressive nature and poor prognosis of NECs and stresses the need to identify patients at high risk of developing NECs and develop improved surveillance guidelines for detecting them.