The incidental finding of a rare appendiceal tumour in a previously healthy male patient is described. He presented as an emergency with a self-inflicted abdominal stab injury. At laparotomy, there was no visceral damage but the appendix was hugely distended and appendectomy was performed. Subsequent histological examination of the appendix revealed a low-grade mucinous cystadenocarcinoma. A right hemicolectomy was performed two months later which demonstrated no residual tumour.