Gangliosides GD3 and GD2 are specifically expressed in neuro-ectoderm-derived tumors, and are considered to play roles in the malignant properties of those cells. We analysed effects of small interfering (si) RNAs against GD3 synthase gene on the expression of ganglioside GD2 and biological phenotypes of human lung cancer cells expressing GD2. An siRNA could suppress the mRNA level of GD3 synthase gene even by single transfection, whereas repeated transfection was required to suppress GD2 expression on the cell surface. Significant reduction in the cell growth and invasion activity was observed in both lung cancer cell lines examined, when repeatedly transfected with the siRNA twice a week. DNA ladder formation was observed after third transfection, indicating the potent induction of apoptosis. Stable transfection of an RNAi expression vector with H1 RNA promoter was also examined. Transfectant cells with the RNAi expression vector showed almost equivalent suppression of GD2 expression and tumor properties in vitro. Furthermore, the stable transfectant cells showed slower cell growth than the control cells in severe combined immunodeficiency mice. These results suggested that siRNAs and/or RNAi expression vectors to generate siRNAs are promising approach to overcome human lung cancers.