Melena is a form of blood in stool which refers to the dark black, tarry feces that are commonly associated with upper gastrointestinal bleeding.

[1] The black color and characteristic strong odor are caused by hemoglobin in the blood being altered by digestive enzymes and intestinal bacteria.

[7] Causes of upper gastrointestinal bleeding that may result in melena include malignant tumors affecting the esophagus, stomach or small intestine, hemorrhagic blood diseases, such as thrombocytopenia and hemophilia, gastritis, stomach cancer, esophageal varices, Meckel's diverticulum and Mallory-Weiss syndrome.

[citation needed] Bleeds that originate from the lower gastrointestinal tract (such as the sigmoid colon and rectum) are generally associated with the passage of bright red blood, or hematochezia, particularly when brisk.

[citation needed] The origin of melena is dated to the early 19th century via modern Latin, via Greek melaina (feminine of melas, black).