Insulin-like growth factor II (IGF-II) plays a key role in mammalian growth, influencing foetal cell division and differentiation and possibly metabolic regulation. The mature 67 amino acid peptide shares sequence homology with both insulin and IGF-I. The liver is the main endocrine source of IGFs, but autocrine/paracrine activity is found in most tissues. The type 1 receptor mediates most of the biological effects of IGF-I and IGF-II; the type 2 receptor is involved with IGF-II degradation. Binding proteins may both localise IGFs to the receptors and regulate their activities. The IGF2 gene is maternally imprinted in mouse and human. Relaxation of IGF2 imprinting occurs in the Beckwith-Wiedemann syndrome of somatic overgrowth, sporadic Wilms' tumour and a number of other cancers. In the general adult population, the IGF2-INS gene cluster may also influence body weight, in which case IGF-II function could become a target for therapeutic intervention in obesity.