By the development of gastrofiberscopy and biopsy procedure, the diagnosis of duodenitis became easier. In general, the endoscopic findings such as hyperemia, erosion or granularity of duodenal mucosa are suggestive of duodenitis. In this study, we compared the endoscopic findings with microscopic changes using duodenal biopsy specimens of 106 patients with duodenitis and 25 healthy people at Korea University Medical Center. In duodenum of healthy people, the ratio of villi length and crypt length was 2.19: 1 and the percentage of lymphocytes and plasma cells among inflammatory cells was 88.6% and neutro- phil only 1%. In duodenitis, the hyperemic type revealed flattening of villi and neutrophil in- filtration and the erosive type revealed rnarkedly decreased length of mucosal villi and promi- nent neutrophil infiltration as main histological alterations. In the mixed type (hyperemia with hypertrophy) of duodenitis, flattening of mucosal villi and increase of crypt depth were the main histological characteristics and in the mixed type (hyperernia with erosion) of duodenitis, there was only decrease of mucosal villi length, but no significant changes in cellularities. The histological changes in the mucosa near the lesion site of duodenitis were similar to the changes in the lesion site. (Korean J Gastroentero11994; 26: 25 38)