Magnetization processes in ultrathin transition-metal films with perpendicular magnetic anisotropy are investigated. By model calculations it is shown that nucleation in ideal films is incoherent and therefore bulklike, whereas the truly ultrathin limit of coherent nucleation is restricted to film patches of small cross-section areas. In ideal monolayers, the nonzero film thickness leads to bulklike nucleation if the lateral dimensions of the film exceed about 1 μm. This means that monolayer patches having submicrometer diameters cannot be regarded as ultrathin in a micromagnetic sense. On the other hand, the critical single-domain diameter of ultrathin films is larger by one order of magnitude than expected from bulk-type thin-film calculations.