The biocompatibility of undoped and n-type silicon nanowires (SiNWs), synthesized by metal-catalyzed chemical vapor deposition, was investigated in vitro, and compared with that for SiO2-coated silicon wafers. In cell interaction studies, L929 mouse fibroblast cells were used, in which MTT assay, AO/PI dye staining, cell proliferation/growth, and changes in cell adhesion/morphology on the surfaces were applied and investigated. Heamocompatibility of these materials were also tested. No significant cytotoxic effects were observed for either the undoped or for the n-type phosphorus-doped SiNWs. The cells on the nanowires exhibit high viability, normal nuclear, and morphological structure. They have shown no considerable negative blood response with respect to the control.