A 46-year-old man was seen with a 2-month history of crampy abdominal pain and recent onset of hematochezia. Colonoscopic examination revealed a semiannular mass lesion in the descending colon which was thought to represent a near-obstructing neoplasm. A partial colon resection was performed. Gross inspection revealed a segment of bowel with focally necrotic mucosa but no mass lesion. Microscopic examination revealed deposits of amyloid infiltrating the muscularis propria, submucosal vessel walls, and lamina propria, with focal ischemic necrosis of mucosa. Special stains were positive for light chains, indicating primary amyloidosis. Follow-up studies for multiple myeloma and inflammatory disorders gave negative results. A diagnosis of isolated amyloidosis of the colon was rendered. Two months after surgery, the patient had a recurrence of symptoms and colon biopsy specimens revealed amyloidosis. The patient was given colchicine, with subjective and objective improvement. The various types of amyloidosis are discussed.