Other than that in the duodenum, adenocarcinoma in the small bowel is rare. The present study describes a case of adenocarcinoma with adenoma in the jejunum. A 70-year-old male was admitted to hospital due to dehydration induced by abdominal discomfort and difficulty with oral intake. Computed tomography revealed a tumor in the upper side of the jejunum, which was subsequently resected. The tumor contained adenocarcinoma and adenoma. The protein expression of p53 and Ki-67 was analyzed in the normal mucosa, adenoma and adenocarcinoma. The number of epithelial cells expressing p53 and Ki-67 was found to increase in the adenoma tissue compared with that in the normal mucosa. In the adenocarcinoma tissue, the number of cells expressing p53 and Ki-67 further increased, suggesting that an adenoma-adenocarcinoma sequence may occur in the small bowel, similar to that observed in the large bowel.