Insulin-like growth factor I (IGF-I) has acute insulin-like metabolic effects and long-term anabolic actions. The therapeutic potential of recombinant human IGF-I treatment is being investigated in various growth hormone-resistant and insulin-resistant disorders. Recent studies have shown that IGF-I may substitute for growth hormone in promoting linear growth in children with growth hormone insensitivity. The anabolic, protein-sparing action of IGF-I is being evaluated as a potential therapy for adults with catabolic diseases. Patients with insulin-dependent diabetes mellitus have reduced endogenous IGF-I production, and studies are in progress to determine whether treatment with IGF-I in addition to insulin may improve their metabolic/anabolic status. Insulin-like growth factor I treatment may reduce glucose and triglyceride levels in adults with non-insulin-dependent diabetes mellitus and in some patients with extreme insulin resistance. Further studies are needed to evaluate the efficacy and safety of IGF-I treatment in these and other conditions and to provide a better understanding of this hormone's normal physiologic role(s) and complex relations with growth hormone and insulin.