Two boys with classical progeria have been studied by a variety of laboratories in the Boston area. Evidence for unresponsiveness to growth hormone, relative insulin resistance, and highly cross-linked collagen is presented. The data support the concept that progeria represents a form of mesenchymal dysplasia in which connective tissue cells are unresponsive to growth influences. It is hypothesized that this failure to grow may account for the "older" collagen which may in turn produce structural abnormalities in vessel walls predisposing to atherosclerosis.