Somatomedins are polypeptide hormones (MW: 7500 Daltons) whose plasma concentrations are largely governed by growth hormone secretion. Somatomedins stimulate cartilage growth and mitosis and growth of several extraskeletal cell types. Somatomedins also display insulin-like activity in adipose tissue. Presently four different human somatomedins are known. Somatomedin C (SmC) and insulin like growth factor I (IGF I) turned out to be identical peptides. TO a large extent they are regulated by growth hormone. Thus they mediate growth hormone action at the tissue level. Insulin-like growth factor II (IGF II) is only minimally dependent on growth hormone secretion. Its definite biological role for growth remains to be established. The somatomedins are bound to larger carrier proteins in the circulation. Somatomedins are synthesized in mesenchymal cells of multiple organs, especially in the liver and kidneys. Somatomedins are of clinical relevance for the diagnosis of growth disturbances due to pituitary disorders. In pituitary dwarfism radioimmunological SmC/IGF plasma levels are decreased whereas in acromegaly they are increased. In a small percentage of patients both with pituitary dwarfism and acromegaly normal SmC/IGF I concentrations are encountered. These facts demonstrate that SmC/IGF I determinations cannot replace common diagnostic procedures in the analysis of growth disorders. The reliability of low SmC/IGF I concentrations is limited in conditions like low-calorie malnutrition, malabsorption, various storage diseases, hypothyroidism, chronic liver and kidney diseases, because in these disorders low SmC/IGF I plasma concentrations occur despite high growth hormone levels.