The clinical significance of osteonectin in human stomach cancer was examined immunohistochemically and molecular biologically in 31 differentiated and eight undifferentiated stomach adenocarcinomas and 19 non-cancer stomach tissues. Osteonectin-mAb-stained cells were observed in stroma of 90% differentiated and 63% undifferentiated adenocarcinomas, and of 26% non-cancer stomach tissues. Competitive reverse transcriptase polymerase chain reaction results generally coincided with immunohistochemical data. The present results suggest that osteonectin is highly expressed in reactive stroma associated with invasive differentiated adenocarcinomas and that it may serve as a useful clinical diagnostic marker for stomach cancer.