It has been assumed that idiopathic megacolon occurring in the spontaneously diabetic BB rat results from metabolic decompensation associated with diabetes. In a large prospective necropsy study we have documented the incidence of idiopathic megacolon and its distribution between diabetic and non-diabetic animals in the BB rat colony in Edinburgh. A significantly increased incidence of megacolon was found in non-diabetic rats in the diabetes-prone subline compared with diabetic rats in the diabetes-prone subline and with non-diabetic rats in the diabetes-resistant subline. Affected animals were found to have an inflammatory infiltrate in the wall of the large bowel with destruction of both autonomic ganglia and muscularis propria. These necropsy observations indicate that megacolon cannot be a result of the metabolic disturbance due to the hyperglycaemia associated with diabetes.