The endoscopic Congo red test allows accurate and rapid evaluation of the completeness of vagotomy and may result in a lower incidence of postoperative incomplete vagotomy. This report describes 44 patients tested during proximal gastric vagotomy. Evidence of incomplete vagotomy was found in over 95% at the conclusion of the conventional operation. Importantly, the test was a guide to further operative maneuvers which abolished the evidence of incomplete vagotomy upon subsequent testing. The endoscopic Congo red test satisfies the requirements for an ideal test for complete vagotomy: it is easily performed, does not require special equipment, and can be repeated several times if necessary to verify that desired effects have been achieved. The wider use of this test, therefore, appears justified.