Excessive health anxiety - which is anxiety about one's health that is disproportionate to the person's medical status - is a common and often debilitating problem. Little is known about its etiology. The present study investigated the role of genetic and environmental factors using a classic twin study method. Results indicated that, after controlling for medical morbidity, environmental influences accounted for most of individual differences in health anxiety. These findings underscore the importance of psychosocial interventions, which have been shown to be among the most effective interventions for excessive health anxiety.