Five children from 3 different families presented with severe dwarfism and the morphological and biological features described by Laron: familial occurrence, small stature, peculiar facies, high levels of plasma HGH and resistance to treatment by GH. This therapeutic inefficiency is expressed by an absence of physical growth and unchanged nitrogen balance, during a prolonged treatment. The plasma levels of somatomedine were very low (K. Hall's biological method) and not influenced by administration of exogenous HGH. These findings suggest that the fundamental disorder is not an abnormal structure of the molecules of GH. This hypothesis seems further confirmed by the structural analysis of plasma HGH, which gave the same results as those of the reference GH.