There is a principal and qualitative difference between using regression results on data from groups of children, and using validated prediction models for individual children. Using accurate models, it is now possible to predict the growth response to growth hormone (GH) treatment in a slowly growing child with GH deficiency (GHD) or in a child with idiopathic short stature (ISS). The growth response to the standard dose of GH can be regarded as a bioassay for GH (i.e. the tissue GH responsiveness) and the information on this growth response can be used for different purposes: to decide about treatment or not, for monitoring, and for adjusting the GH dose in order to reach a defined goal for height. This last concept is now used in an ongoing prospective randomized GH dose-finding trial.