Hepatocyte growth factor activator inhibitor type-1 (HAI-1) is an integral-membrane proteinase inhibitor. In this study, we examined the effects of HAI-1 on human glioblastoma cells. Two glioblastoma cell lines (YKG-1, U251) were stably transfected with expression plasmid harboring mature membrane-form or truncated secreted-form HAI-1. Culture characteristics were not altered by the expression of HAI-1, whereas in vitro invasiveness of U251 was suppressed. On the other hand, the expression of membrane-form HAI-1 resulted in significantly enhanced tumorigenicity of both cell lines in vivo. In contrast, secreted-form HAI-1 did not promote the tumorigenicity. These results suggest that HAI-1 may play complex roles in progression of glioblastoma cells, and membrane-form HAI-1 may mediate an undefined important signaling in the cells.