A 29-year-old female visited a hospital because of increasingly severe lower leg edema. She was diagnosed as having multiple polyps in the stomach and colon by gastroscopy and sigmoidoscopy as well as multiple liver tumors by abdominal CT. She was referred to our hospital for further examination. Total colonoscopy revealed a type 2 tumor in the transverse colon and more than 200 polyps distributed throughout the colorectum. Biopsies of the tumor and polyps showed histological characteristics of adenocarcinoma and tubulovillous adenoma, respectively. Thus, she was diagnosed as having metastatic colon cancer derived from familial adenomatous polyposis (FAP). Laboratory tests showed a marked hypoalbuminemia of 1.1 g/dl. The fecal alpha-1 anti-trypsin test showed abnormal clearance (62.1 ml/day), and scintigraphy using 99mTc-human serum albumin revealed protein loss in the whole colon. Multiple ligation probe amplification analysis of the APC gene identified a germline duplication of exons 11-13. Direct sequencing of the reverse transcription PCR products of APC mRNA revealed a deletion of 25 base pairs and a tandem duplication of exons 11-13. This case was considered to be protein-losing enteropathy resulting from numerous colonic tubulovillous adenomas and advanced colon cancer in a FAP patient with unusual mutational events in APC.