Highly branched cyclic dextrin derivatives (CH-CDex) that are partly substituted with cholesterol groups have been synthesized. The CH-CDex forms monodisperse and stable nanogels with a hydrodynamic radii of approximate to 10 nm by the self-assembly of 4-6 CH-CDex macromolecules in water. The CH-CDex nanogels spontaneously trap 10-16 molecules of fluorescein isothiocyanate-labeled insulin (FITC-Ins). The complex shows high colloidal stability: no dissociation of trapped insulin is observed after at least 1 month in phosphate buffer (0.1 M, pH 8.0). In the presence of bovine serum albumin (BSA, 50 mg center dot mL(-1)), which is a model blood system, the FITC-Ins trapped in the nanogels is continuously released (approximate to 20% at 12 h) without burst release. The high-density nanogel structure derived from the highly branched CDex significantly affects the stability of the nanogel-protein complex.