The objective of this investigation was to develop models describing the vertical variation in internal knot size of Scots pine (Pinus sylvestris L.) stems. The sawlog portion of 198 mature trees was scanned using computed tomography in order to measure internal knot size. The variation in knot diameter at the sound/loose knot border was described in terms of a non-linear segmented (hyperbolic-quadratic-quadratic) model, and as a function of stand variables, tree variables and height above ground. A second, simpler (hyperbolic-quadratic) model which was deemed easier to implement was also presented. Although no validation data set was available, the models performed adequately for trees with different growth rates, growing on different sites and being at different stages of development. These results are intended to be integrated in a model system for conversion simulation studies applicable to mature trees.