A 30-year-old woman was referred because of increasingly severe constipation and a left para-ovarian tumor. She presented initially at 25 years of age because of nausea during menses, and a tumor near the left ovary was found. Abdominal computed tomography showed severe inhibition of bowel motility and a tumor (70 × 30 mm) near the left ovary. On colorectal endoscopy, a submucosal tumor was found in the rectum, 6 cm from the anal verge, and a biopsy showed carcinoid tumor. Transanal resection of the rectal tumor and simple resection of the para-ovarian tumor were performed. The 1.2-cm-diameter rectal tumor in the mucosa and submucosa showed the typical histology of a carcinoid tumor. On immunohistochemistry, approximately 70% of tumor cells were peptide YY (PYY) positive, and Ki-67–positive nuclei accounted for less than 1%. The para-ovarian mass was an encapsulated solid tumor measuring 65 × 35 × 30 mm, and was histologically and immunohistochemically similar to the rectal tumor. Serum PYY was more than 8 times higher than that of control subjects preoperatively; it fell to half postoperatively, and her symptoms subsided. However, 111In-pentreotide scintigraphy showed somatostatin receptor–positive tumors located near the rectum and left obturator cavity. On laparoscopy, 2 tumors, 2.5 and 1.6 cm in diameter, in the mesorectum and a 1.0-cm-diameter tumor in the obturator canal were resected. Histologically and immunohistochemically, the tumors were similar to the previously resected tumors except for the lymph node found in 1 tumor. Based on this case, severe constipation due to PYY appears to be a new carcinoid syndrome.