Specific siRNAs that target estrogen receptor alpha (ER alpha) were encapsulated in nanocapsules (NCs). We produced small (similar to 100-200 nm) ER alpha-siRNA NCs with a water core by incorporating two mixed duplexes of specific ER alpha-siRNAs (ER alpha-mix-siRNA) into NCs. The encapsulation yield that was obtained with poly(isobutylcyanoacrylate) (PIBCA) NCs was low, whereas no release of trapped siRNA was observed for poly(ethylene)glycol-poly(D,L-lactide-co-glycolide) (PEG-PLGA) NCs. High levels of ER alpha-siRNA incorporation into PEG-epsilon-caprolactone-malic acid (PEG-PCL/MA) NCs (3.3 mu M in a polymer solution at 16 mg/mL) were observed (72% yield). No difference in size or zeta potential was observed between siRNA NCs that were based on PEG-PCL/MA and empty NCs. Fluorescence quenching assays confirmed the incorporation of siRNA into the NC core. A persistent loss of ER alpha (90% over 5 days) was observed in MCF-7 human breast cancer cells that were exposed to PEG-PCL/MA NCs that were loaded with ER alpha-siRNA. The intravenous injection of these NCs into estradiol-stimulated MCF-7 cell xenografts led to a significant decrease in tumor growth and a decrease in ER alpha expression in tumor cells. These data indicate that a novel strategy, based on ER alpha-siRNA delivery, could be developed for the treatment of hormone-dependent breast cancers.