Mixed endocrine-exocrine carcinoma of the appendix is a rare tumor which exhibits histopathologic features of both adenocarcinoma and an endocrine tumor. It is well known that, clinic course of these tumor is more aggressive than conventional endocrine tumor.

A 45-year old man complaining of abdominal pain and fever, underwent appendectomy with the preliminary diagnosis of acute abdomen. During gross examination of appendectomy specimen, a perforation area with 0.6 cm in diameter was determined. Microscopic examination of the cross-sections of this perforated area revealed tumoral infiltration beneath the surface epithelium attacking all layers of the appendix. The tumoral infiltration is composed of atypic cells, forming small glandular structures with Goblet cells with a benign appearance, and tending to nest with its large nucleus and eosinophilic cytoplasm in desmoplastic stroma. Intra and extracellular mucin accumulation is determined within the tumor with mucicarmine reaction. Mitotic count was ≤3 in 10 high power field.

Immunohistochemically, tumor cells showed diffuse reaction with ephitelial membrane antigen, choromogranin A, carcinoembrionic antigen and reactivity with p53 (5-10 %). With respect to these findings, the diagnosis was 'mixed endocrine-exocrine carcinoma'. Due to malignant potential of the tumor, after an intense investigation for any focus, the patient underwent to right hemicolectomy. At 5 months of follow -up postoperatively, neither metastasis nor any recurrence was observed.

This case is reported for its rarity and malignant potential.