We have performed a prospective study on 117 patients in order to assess the value of a single contrast water soluble enema in acute large bowel obstruction. In 99 cases where the plain films suggested a diagnosis of mechanical obstruction, the Diodone enema confirmed its presence in 52, and relocated its site in 11. There was free flow of contrast to the caecum in the remaining 35, 11 of whom were shown to have idiopathic pseudo-obstruction. The plain films suggested a diagnosis of pseudo-obstruction in 18 patients. The Diodone enema confirmed this diagnosis in 15, but revealed an unsuspected mechanical obstruction in two. The examination failed in two cases. We conclude that the water soluble single contrast enema is of value in the management of large bowel obstruction. It will confirm the diagnosis of mechanical obstruction and prevent unnecessary surgery in patients with pseudo-obstruction.