Four cases of adenocarcinoid of the appendix were studied. Two tumours were found among 28 cases primarily diagnosed as appendiceal carcinoids. They showed characteristic histological structures with features of both a conventional carcinoid tumour and a mucinproducing adenocarcinoma with goblet cells. All tumours were small and ill-defined; three were associated with fibrous obliteration of the appendiceal lumen. All were diagnosed incidentally by the pathologist in appendices removed "en passant" or because of acute appendicitis. Three of the tumours appeared well differentiated with a low degree of malignancy similar to that of the conventional carcinoid tumour. In one case however, the tumour was less differentiated with atypical foci and a high mitotic count and had metastasised to peritoneum and both ovaries.