We report herein a rare case of diverticulitis causing a high serum level of carbohydrate antigen (CA) 19-9. A 52-year-old man was admitted to our hospital with lower abdominal pain. Laboratory data showed evidence of inflammation and a high serum level of CA 19-9 (370 U/ml). Computed tomography demonstrated thickening of the wall of the sigmoid colon. He was diagnosed as having diverticulitis of the sigmoid colon and was treated with antibiotics. Although his symptoms improved, the presence of a malignancy such as colorectal cancer could not be completely ruled out because of the persistently high serum level of CA 19-9. A laparotomy was performed and the sigmoid colon was found to be adherent to the bladder. Under a diagnosis of diverticulitis, a sigmoidectomy was performed. Pathological examination revealed diverticulitis of the sigmoid colon, but there was no evidence of malignancy in the resected specimen. The serum CA 19-9 level decreased to normal postoperatively and immunohistochemical staining revealed CA 19-9 antigen in the cytoplasm of the diverticular epithelium. Therefore, a possible explanation for the high level of this tumor marker was diverticulitis of the sigmoid colon.