Primary peritoneal cystadenocarcinoma is a rare tumor of similar histogenic origin as primary ovarian carcinoma. We present a case of primary peritoneal serous cystadenocarcinoma mimicking advanced colorectal cancer in a 68 yr-old African American female. Radiology, endoscopy and cytology yielded only inconclusive findings. Immunohistochemical analysis of percutaneously obtained ascitic fluid provided a correct diagnosis of primary peritoneal cystadenocarcinoma. The discovery of serous ascites at the time of laparotomy confirmed a diagnosis of primary peritoneal serous cystadenocarcinoma. Final surgical pathology reconfirmed the diagnosis of primary peritoneal cystadenocarcinoma. This case demonstrates the utility of immunohistochemistry for accurately diagnosing patients with inconclusive findings in the setting of peritoneal carcinomatosis and primary peritoneal cystadenocarcinoma.