Star-branched microgels (SBM's), prepared by the reaction of living poly(methyl methacrylate) chains with ethylene glycol dimethacrylate, were characterized by static and dynamic light scattering, viscometry, and size-exclusion chromatography with a low-angle laser light scattering detector. The average number of arms per microgel, the arm length, the size of the microgel, the molecular weight distribution, and the frequency distribution of arms per SBM were determined. The influence of branching on the shrinking factors of the SBM's relative to linear polymers was found to deviate, as expected, toward hard-sphere behavior. After correction for polydispersity, qualitative agreement with regular star polymers was found for the interdependence of the radius of gyration and intrinsic viscosity shrinking parameters. Unexpectedly, it was found that the ratio of the hydrodynamic volumes of SBM's to linear polymers at the same molecular weight is dependent on the number of arms but independent of the arm molecular weight.