When acute perforation of the colon is suspected in a patient, a barium enema examination is obviously contraindicated. However, an unsuspected colon perforation or its occurrence during the administration of a barium enema may permit the free escape of barium fluid into the peritoneal cavity. This is probably not as rare an incident as would be indicated by the paucity of reported cases. The radiologist should be aware of this possibility and alert to recognize its roentgenoscopic appearance should it occur in spite of precautions. The cases of two patients with perforation of the sigmoid colon are reported in this paper. In both instances the roentgenoscopic and radiographic findings were strikingly similar and presented a particular sign that may be of value for the prompt recognition of such a perforation during the roentgenoscopy. The barium escaping through a perforation in the sigmoid may immediately collect in the space between the