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Introduction: Primary appendiceal adenocarcinoma is rare (less than 250 cases described in the literature). In this article a case of appendiceal carcinoma with peritoneal seeding is reported. Case: The patient is a 38 years old man presented with pain, tenderness and rebound in the lower quadrant of the abdomen. These findings suggested acute appendicitis. Emergency laparotomy showed inflamed appendix, without adhesion to the surrounding tissue or perforation. Appendectomy was performed. The histologic diagnosis revealed a moderately differentiated appendiceal adenocarcinoma. The tumour had infiltrated up to serouca of the appendix. The patient was reffered to oncology center but due to refusing of patient for further evaluation, six months later he was admitted with dyspnea, abdominal pain and prorusion. Clinical examination and diagnostic work ups showed massive exudative ascites. The patient was admitted to the oncology ward of Vali-e-Asr hospital with the diagnosis of appendiceal adenocarcinoma with peritoneal involvement. Conclusion: In case of acute appendicitis, the possibility of appendiceal adenocarcinoma should be considered