In the present study, the completeness of vagotomy in patients who underwent truncal and highly selective vagotomy is evaluated, comparing them with control groups of normal persons and patients with a duodenal ulcer. The evaluation was made by the Hollander test, followed by the endoscopic Congo red test, using the same hypoglycemic stimulus. Gastric acid production was higher in the patients with duodenal ulcer, followed by the normal persons. Lower acidity levels were obtained for patients with truncal vagotomy than for patients with highly selective vagotomy. During the Congo red test, we noted that the patients with truncal or highly selective vagotomy had small black spots in the gastric mucosa, except those having a recurrent ulcer, denoting incomplete vagotomy. Nevertheless, when we observed the appearance of the gastric mucosa, we noted a similar aspect for those having had a vagotomy and for normal persons. This appearance was completely different from the patients with a duodenal ulcer. We concluded that truncal vagotomy greatly reduces the level of gastric acid production, while highly selective vagotomy does so to a lesser extent. The endoscopic aspect of these patients during the Congo red test was similar to that for normal persons.