We report a series of 13 lesions of the human vermiform appendix in wh
ich a carcinoid component was associated with a separate non-carcinoid
epithelial component that included an adenoma-like lesion of the muco
sal epithelium. We use the term dual carcinoid/epithelial neoplasia to
describe this phenomenon. The carcinoid component was insular/trabecu
lar in nine cases, tubular in one case and of goblet cell type in thre
e. The epithelial component was a mucinous cystadenoma in four, a muci
nous tumour of uncertain malignant potential in three, and a mucinous
cystadenocarcinoma in six, No intermediate cell population was seen an
d in three cases the carcinoid and epithelial components were in diffe
rent parts of the appendix, leading us to suggest that these lesions m
ay be true 'collision' tumours in which two neoplasms have arisen in t
he same organ, The prognosis appears to be no worse than for either of
the components alone, but conclusions regarding these lesions must be
guarded on account oi. their rarity and the small numbers available f
or study.