The extent to which nanoscale-engineered systems cross intact human skin and can exert pharmacological effects in viable epidermis is controversial. This research seeks to develop a new lipid-based nanosome that enables the effective delivery of siRNA into human skin. The major finding is that an ultraflexible siRNA-containing nanosome-prepared using DOTAP, cholesterol, sodium cholate, and 30% ethanol-penetrates into the epidermis of freshly excised intact human skin and is able to enter into the keratinocytes. The nanosomes, called surfactant-ethanol-cholesterol-osomes (SECosomes), show excellent size, surface charge, morphology, deformability, transfection efficiency, stability, and skin penetration capacity after complexation with siRNA. Importantly, these nanosomes have ideal characteristics for siRNA encapsulation, in that the siRNA is stable for at least 4 weeks, they enable highly efficient transfection of in vitro cultured cells, and are shown to transport siRNA delivery through intact human skin where changes in the keratinocyte cell state are demonstrated. It is concluded that increasing flexibility in nanosomes greatly enhances their ability to cross the intact human epidermal membrane and to unload their payload into targeted epidermal cells. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.