Administration of immune cells to cancer patients may require large cell numbers of sufficient purity. In addition, for effective cellular immune therapy, ex vivo manipulation of the cells before administration to patients may be desirable. NK cells are a specific class of immune cells with promising tumor-killing capacity. However, it has remained challenging to obtain sufficient cells from donors for immune therapeutic purposes. An ex vivo expansion technology was developed and filed for patent registration allowing the > 1.000x expansion of pure NK cell populations from immature hematological stem cells derived from umbilical cord blood. This NK cell product was successfully tested in a phase I clinical trial in leukemia patients showing feasibility and safety. Currently phase I/ II clinical trials are in development for leukemia and ovarian carcinoma patients.