The occurrence of large-bowel melanosis was evaluated by microscopy in 200 large bowels at autopsy. Melanin was seen as yellow-brown pigment in the macrophages of the lamina propria. The pigment stained with diastase-alcian blue PAS, Fontana, and iron stains. One hundred nineteen of 200 (59.5 percent) bowels showed melanosis, which was equally common in both sexes. Usually more than one segment was involved (most commonly, four segments). Melanosis was common in the proximal part of the colon, but much rarer in distal parts (sigmoid and rectum). Affected segments were successive; negative segments between positive ones were exceptional. If the rectum was affected, all five proximal segments were affected in 11 of 12 cases. The intensity of melanosis was directly related to the number of segments involved. In the oral part of the colon, affected males had a higher intensity of melanosis than affected women, but about the same intensity in the sigmoid and rectum. The fraction of patients with melanosis increased with age. Of men and women in the age group of 20 to 54 years, 32 and 44 percent were affected, and above the age of 75 years, 76 and 67 percent, respectively.