The study of illness cognition and behavior has relied primarily on nonexperimental research designs. In this paper we review the results of a program of experimental investigations of psychological reactions to health threats. Most of the studies employ a new experimental paradigm developed to study illness cognition and behavior in the laboratory. The paradigm has been used in several experiments to examine the cognitive and motivational processes underlying reactions to medical test results. A converging series of studies has shown that denial, manifested in a variety of ways, is a common initial reaction to threatening information. In addition, the studies demonstrate that the perceived prevalence of a health disorder is an important determinant of its perceived seriousness. The insights gained from these studies illustrate the complementary roles of basic experimental research and more naturalistic observational research in the formulation of comprehensive theories of health and illness behavior.