Seventeen cases of primary duodenal adenocarcinoma occurring in the elderly (older than 65 years) were examined to clarify their clinicopathological features and biological behavior. The mean age was 77.4 years (range, 66-104), and there was no appreciable difference in the incidence between the sexes (female: male ratio, 8: 9). Thirteen patients had tumors located in the first portion of the duodenum, three in the second portion and one in the third portion. Grossly, there were three varieties of lesions: six polypoid, four flat-elevated and seven ulcerative-invasive. Sixteen cases showed well-differentiated adenocarcinoma, three of which were difficult to distinguish from adenoma, and one was poorly differentiated adenocarcinoma. Three of 16 tumors had poor differentiation in the invasive area, whereas mucosal lesions were well differentiated. Eight tumors had invaded the duodenal wall with occasional involvement of the pancreas. Immunohistochemistry demonstrated p53 protein overexpression in two intramucosal (22.2%) and five invasive (62.5%) cancers. In the intramucosal area the mean Ki-67-positive rate (PR) of the tumors with distant metastasis was significantly higher than that of the tumors without metastasis (46.0 vs 31.6%; P < 0.05), while there was no significant difference in the association between PR and gross feature or depth of the tumors. Clinical follow-up showed three of the five patients with invasive cancer died of carcinoma within 28 months. Compared with published data from other investigators, the results of the present study indicate a proximal shift of duodenal carcinoma in the elderly. Furthermore, it is concluded that invasive duodenal adenocarcinomas with high PR should be considered as potentially aggressive tumors, although their histology may indicate a high degree of differentiation.