The invention relates to the treatment and/or prevention of tumor diseases associated with cells expressing CLDN6, in particular cancer and cancer metastasis using antibodies which bind to CLDN6. The present application demonstrates that the binding of antibodies to CLDN6 on the surface of tumor cells is sufficient to inhibit growth of the tumor and to prolong survival and extend the lifespan of tumor patients. Furthermore, binding of antibodies to CLDN6 is efficient in inhibiting growth of CLDN6 positive germ cell tumors such as teratocarcinomas or embryonal carcinomas, in particular germ cell tumors of the testis.