Gastrointestinal duplications are an uncommon congenital abnormality that manifest before the age of two in 80% of cases. Heal duplication is the most common while colonic duplication, either cystic or tubular, occurs in 10%-15% of cases and remains asymptomatic and undiagnosed in most cases. Mostly occurring in pediatric patients, colonic duplication is encountered in adults in only a few cases. The most common clinical manifestations are abdominal pain and intestinal obstruction. Rarely, duplications present with signs of acute abdomen or acute bleeding. This study reports a case of colonic duplication in an adult who presented with chronic constipation. Complete diagnostic workup was made on several occasions during the previous eight year period, but no pathology was found and chronic constipation was attributed to hypothyroidism caused by long standing Hashimoto thyroiditis. Multislice CT, performed because of abdominal distension, defined colonic pathology but the definite diagnosis of duplication of the transversal colon was made at operation. The cystic duplication and the adjacent part of the ascending and transversal colon were excised en-block. This study implies that colonic duplication, though uncommon, should be included in the differential diagnosis of chronic constipation even when precipitating factors for constipation, such as hypothyroidism are present.