Goblet cell adenocarcinoma is a malignant tumour that arises in the appendix. The tumour is composed of goblet-like mucinous cells, endocrine cells, and Paneth-like cells. Grossly the appendix may appear normal or slightly thickened. Microscopically the tumour may exhibit low- and high-grade features, and the relative proportion of each determines the overall grade. Low-grade features include a tubular architecture resembling intestinal crypts with no stromal response, mild atypia, and infrequent mitotic figures. High-grade features include cribriform growth, single-cell infiltration, signet-ring cells, nuclear atypia, increased mitotic figures, and a desmoplastic stromal reaction. Extracellular mucin is almost always present. By immunohistochemistry, the tumour cells are positive for CK20, chromogranin, and synaptophysin.