This review describes the recent developments in the diagnosis and treatment of states of resistance to growth hormone based mainly on the studies performed on patients with an inborn molecular defect of the growth hormone receptor, ie, Laron syndrome. Use of the polymerase chain reaction technique has facilitated the study of the defect, which in the large majority of patients resides in the extracellular domain of the growth hormone receptor. This recessively transmitted hereditary disease, characterized clinically by dwarfism and obesity and biochemically by high levels of serum human growth hormone accompanied by low serum concentrations of insulin-like growth factor I, is diagnosed mainly in inbred societies of Asian Jews, Arabs, Iranians, Italians, and Spaniards or their descendants. Isolated mutations have been found in many countries all around the world. The recent biosynthesis of insulin-like growth factor I has enabled the successful treatment of these patients, accelerating their growth, reducing their obesity, and normalizing their metabolic abnormalities resultant from the insulin-like growth factor I deficiency. The Pygmies are an inbred population in Africa, Asia, and Oceania who seemingly also have a defect in the growth hormone receptor. Acquired states of growth hormone resistance are also discussed.