Question: Medulloblastoma is the most common malignant brain tumor in the pediatric population. Although conventional treatments improve survival, the chances of recovery remain low and result in severe neurological sequelae, especially in young children. CD3-CD56 + Natural Killer cells are able to recognize and kill tumor cells, particularly during the loss of class I MHC molecules. Their cytotoxic potential has already been demonstrated in vitro and in vivo with certain brain tumors but few studies concern medulloblastoma. Our aim was to evaluate the cytotoxic effect of allogeneic NK cells on medulloblastoma cells. Methods: Three lines of medulloblastoma subgroups (DAOY, D283 and D341) were analyzed and compared to the K562 leukemia cell line, known to be very sensitive to NK cells. The NK cells were selected from blood of healthy donors by immunomagnetic selection, and amplified in the presence of IL-15. After expansion, their cytotoxic potential was evaluated by different tests targeting each step involved in the process of tumor lysis: the phenotype of tumor cells by flow cytometry (expression of B7 molecules, CD56, 73, PVR and MICA/B), conjugation by flow cytometry and confocal microscopy, the degranulation by granzyme B elispot and finally the death of the target cells, by necrosis, late apoptosis and cell lysis (by flow cytometry and adenylate kinase measurement). The impact of co-culture with tumor cells on the expression of inhibitory or activating receptors was also analyzed. Nk cells were next injected in medulloblastoma xenografted nude mice and the survival was followed. Results: 3 medulloblastoma lines expressed CD56, PVR and CD105. The expression was heterogeneous for B7H1, H3 and H4, CD73, and MICA/B. The 3 lines were able to conjugate with the NK cells and to induce their degranulation. A necrosis percentage of between 30% (D283 and D341) and 38% (DAOY) could be measured after 4 hours of incubation. Percentages of cell lysis between 32 and 77% could be detected for the DAOY and D341 lines respectively. After co-culture, an increase in expression of the activating receptors NKG2D and DNAM1 could be observed while the expression of the inhibitory receptors KIR2DL1 and 2DL5 remained stable. These results suggested that NK cells were able to exert effective cytotoxicity against medulloblastoma. The in vivo survival study of medulloblastoma xenografted nude mice showed an increased survival of the NK cell treated group.