Primary carcinoma of the small intestine is rare and represent about 0.5% of all gastrointestinal malignancies. The aim of this study was to examine the biological characteristics of primary carcinoma of the small intestine by immunohistochemical and nested polymerase chain reaction methods. Thirty-five primary carcinomas (12 in the duodenum and 23 in the jejunum or ileum) from 35 patients were studied clinicopathologically and examined for overexpression of p53 protein. In 22 of these 35 cases, point mutation at codon 12 of the K-ras gene was detected by the nested polymerase chain reaction and restriction fragment length polymorphism method. All the duodenal carcinomas were well-differentiated type and the rate of these carcinomas was significantly higher than that of jejunal or ileal carcinomas (100% vs. 65%). Fourteen cases showed overexpression of p53 (40%), and p53 tended to be expressed more frequently in poorly-differentiated type (71%) compared to well-differentiated type (30%). Only 2 out of 22 carcinoma cases showed K-ras gene mutation, and both were duodenal carcinomas. These findings suggest that p53 plays a major role in the progression of carcinoma of the small intestine, whereas the role of K-ras mutation is much less significant.