Early and serial endoscopic examinations were performed in patients with head injury to determine the actual incidence and course of the acute gastroduodenal lesions. Acute gastroduodenal lesions were found in high incidence of 35 in 47 patients (75%). It was stressed that the lesions were found in 16 of 25 cases without obvious gastrointestinal bleeding. The lesions were mainly found in the stomach (78%) and of these lesions erosive gastritis was seen in the greatest frequency. Gastric ulcer was found in 11 patients. Erosive gastritis was found within one week after the trauma, while gastric ulcer usually developed after one week. Erosive gastritis was mainly located in the body of the stomach and usually healed within ten days. Gastric ulcer also had a good course. There was no case in which the erosive lesion extended to the ulcer. Among 22 patients with gastrointestinal bleeding, three had intractable bleeding. Steroid administration seemed to have little effect on the occurrence of acute gastroduodenal lesions.