The advent of the use of reagents for measuring changes in serum insulinlike growth factor-I and the insulinlike growth factor binding proteins by sensitive techniques has resulted in multiple studies of both regulation of these substances in normal physiologic situations and their pathophysiologic alterations. These tests are now being utilized to determine not only the range of their variations in normal physiologic situations but also the utility of abnormal values in diagnosing disease states. To date, the tests have been used to diagnose growth hormone deficiency and excess as well as to monitor growth hormone replacement therapy. However, because secretion of insulinlike growth factor-I is also altered in diabetes, the diabetic state and responses to insulin therapy have received recent emphasis. Likewise, tumors that produce insulinlike growth factor-I or II, or forms of the insulinlike growth factor binding proteins, and cachectic states such as cirrhosis, AIDS, and burns have also been studied. These data are now well enough formulated for some conditions that we can begin to make predictions about the usefulness of these tests in medical diagnosis.