First-principles calculations using the full-potential linear-muffin-tin-orbital technique have been performed to determine the energy barriers for adatom homodiffusion on the (100) and (111) surfaces of Ag, Au, and Ir. Our results agree very well with the measured energy barriers (when available), i.e., to within 0.03 eV, thereby confirming the adequacy of the theoretical method. On the (111) surfaces, we find that the barriers for Ag and Ir have values that are close to those corresponding to the melting point of the bulk materials, and conclude that ‘‘correlated jumps’’ should be present at high temperatures on these surfaces. For Au(111), on the other hand, the barrier is about twice as large as the melting temperature, and the random-walk model should provide an accurate description of the diffusion process, just as on the (100) surfaces, where the barriers are much larger. Semiempirical models are found to reproduce the first-principles energy barriers within 0.2 eV, which, in some cases, means errors as large as 90 %.