BACKGROUND Primary adenocarcinomas of the appendix are uncommon, constituting 1% of all colorectal malignancies. Appendiceal malignancies often present atypically, thus creating diagnostic challenges. Although there are many pathology reports of primary cystadenocarcinoma of the appendix, only a limited number of cases have appeared in the radiological or surgical literature. We present a unique case of primary cystadenocarcinoma of the appendix occurring concurrently with adenocarcinoma of the colon, and overview the clinical and therapeutic difficulties posed by this rare entity. CASE PRESENTATION A mucocele of the appendix, due to mucinous cystadenocarcinoma, was documented as an incidental perioperatory finding in a 68-year-old female. The patient was admitted due to rectal haemorrhage and underwent colonoscopy with biopsy, X-ray, abdominal ultrasonography and CT scan. Degenerated adenomatous polyp of the ascending colon and mucinous adenocarcinoma of the sigmoid colon invading the parietal peritoneum of the uterine and vagina was diagnosed. At laparoscopy, a cystic appendiceal lesion was found, without perforation. The patient underwent right hemicolectomy, sigmoidectomy and hysterectomy associated with salpingo-oophrectomy. CONCLUSIONS Preoperative diagnosis of an underlying malignancy in a mucocele is important for patient management, but is difficult to reach by imaging studies alone. Synchronous colon cancer may occur in patients with appendiceal mucoceles. In such patients, the colon should be investigated. Surgery is the recommended method of treatment.