The effects of evaluating milk yield in the first three lactations by a single-trait animal model or by a repeatability animal model instead of by the true multitrait animal model were investigated using stochastic simulation. The models were compared both in terms of how accurately they predicted genetic trend when applied to the same dataset and regarding difference in true genetic progress when selecting on breeding values predicted by these models. A breeding structure resembling the Icelandic dairy cattle population was used for the simulation.