Concerning the biological properties of recurrent ovarian cancer, other than drug resistance, we revealed that the expressions of mutant p53 and CD44v6 genes were significantly greater in recurrent ovarian cancer than in those of its primary counterpart. These findings suggest that chemotherapeutic agents may modify some biological characteristics of cancer by altering gene expressions. The biological behavior concerning the metastatic potential of a recurrent disease must be elucidated in order to develop an optional treatment regimen against recurrent tumors. Therefore, we established in-vivo cisplatin-resistant cell lines by repeated administration, in order to find a more suitable model for reflecting the biological aggressiveness of clinically recurrent ovarian cancer following chemotherapy. Chemotherapeutic agents have given a substantial advantage to cancer patients. It must be borne in mind that the cancer cells surviving following chemotherapy possibly present different biological properties from primary cancer cells, and that these properties might be developed by the chemotherapeutic agents.