Islet-amyloid deposits, which are a common feature of Type II diabetes mellitus, are derived from the polymerization of a putative hormone identified as IAPP. IAPP is synthesized by normal islet beta cells and probably is cosecreted with insulin. Although the physiologic function of IAPP and its role in the pathogenesis of Type II diabetes mellitus are just beginning to be unraveled, IAPP may play an important part in the development of this most common form of diabetes mellitus by opposing the action of insulin in peripheral tissues. The polymerization of IAPP to form extracellular islet-amyloid deposits may further contribute to the development of Type II diabetes mellitus by destroying islet cells and by disrupting the passage of glucose and hormones to and from them. Substantial evidence indicates that the propensity of IAPP to polymerize and form extracellular amyloid deposits in only certain species (e.g., humans, cats, and raccoons) is directly associated with an intrinsically amyloidogenic part of the molecule--i.e., positions 20 through 29 of IAPP. The inherent amyloidogenicity of IAPP in these species may be further facilitated by increased beta-cell production of IAPP, leading to a high local concentration that predisposes to polymerization. The latter possibility is supported by studies demonstrating that IAPP production by islet beta cells is increased in normoglycemic cats with impaired glucose tolerance. Although increased production of IAPP may initially cause insulin resistance, prolonged overproduction of IAPP may ultimately impair insulin secretion by leading to the progressive deposition of insoluble islet amyloid, a finding apparent in most subjects with overt diabetes. If, as these studies suggest, increased IAPP production is linked to the development of Type II diabetes mellitus, further studies must address the genetic and nongenetic factors that influence this important biologic change in humans and some animal species.