Clinical investigations started in 1958 of a group of children with characteristics resembling GH deficiency, but who had extremely high levels of plasma GH, led to the description of the syndrome of primary GH resistance or insensitivity (Laron syndrome), the discovery of its molecular defect, and the clinical application of biosynthetic insulin-like growth factor-I. This syndrome is a unique model that enables study of the GH receptor, its signal transduction, and the comparison between the effects of GH and insulin-like growth factor-I.