Nineteen children were studied because of short stature. They had in common abnormally low Sm-C values for age, and each received a ten-day course of exogenous GH therapy. Based on their endogenous GH concentrations and the response to GH therapy, in terms of Sm-C and height increments, they were classified into three groups. Group I included patients with GH insufficiency who had blunted GH responses to stimulation, but responded to therapy by normalizing the Sm-C concentration and velocity of growth. Group II patients all had normal GH responses to stimulation, but their responses to exogenous GH were similar to those observed in the GH deficient subjects. In the two children in Group III who had normal release of endogenous GH, Sm-C values and growth rate did not increase in response to GH. Group II patients may represent children with biologically inactive but immunoreactive GH, whereas the children in Group III are examples of the Laron type of dwarfism. Thus, rather than the plasma GH response to provocative stimuli, the Sm-C and growth increment responses to short-term exogenous GH therapy may more precisely identify children that will benefit from long-term GH therapy.