The Id family of helix-loop-helix proteins is involved in a variety of biological processes, including development,proliferation, and angiogenesis. There is emerging evidence for deregulation of Id proteins in some cancers. Hence we determined Id expression in gastric cancer. To elucidate the involvement of Idl in gastric carcinogenesis, Immunohistochemistry was carried out on paraffin-embedded gastric cancer sections. By analyzing the relationship between the Idl staining index of individual tumors and the patient's clinical parameters, including histological grade, clinical stage, and presence of metastases, we found that strong Idl expression was associated with poorer differentiation and more aggressive behavior of tumor cells. To further explore Idl expression in human gastric carcinomas, Western blot analysis was performed to analyze Idl protein expression in 15 pairs of gastric tissues and cancers and in gastric cell lines. Idl was found to be expressed at higher levels in 11/15 cancer tissues compared to adjacent tissues. Idl was upregulated more prominently in the poorly differentiated cell lines than in the well-differentiated ones. Semiquantitative RT-PCR was carried out to investigate the expression of mRNA of Idl in gastric cancer cell lines and it was upregulated in most of them. Our results demonstrate that Idl protein may play an important role in the process of gastric carcinogenesis and high-level Id I expression may be related to the malignant potential of tumor cells.