The defect energies for the intrinsic and extrinsic stacking faults and for the twin fault in Cu, Ag, and Au were calculated by supercell calculations based on the ab initio mixed-basis pseudopotential method and the linear-muffin-tin-orbital method in atomic-sphere approximation. The calculations correctly reproduce the experimentally observed trend, i.e., very low fault energies for all noble metals, being lower by a factor of about 2 for Ag than for Cu and Au. The physical mechanism (s-d hybridization) is discussed and illustrated by charge-density plots.