The leading contemporary cognitive-behavioral model of excessive health anxiety (HA) emphasizes the importance of environmental factors, such as learning experiences. The model has little to say about the role of genetic factors and, by ignoring these factors, seems to imply that they are unimportant. In contrast, results from the University of British Columbia Twin Study, using a sample of 88 ... [Show full abstract] monozygotic and 65 dizygotic twin pairs, indicated that various facets of HA, such as excessive disease fear, unrealistic beliefs that one has a serious disease, and HA-related interference in functioning, are moderately heritable. The present study extended the analyses of this data set by investigating the extent to which the various facets of HA are due to genetic or environmental factors that are common to all facets versus specific to each facet. Results indicated that all facets of HA are influenced by a common set of genes--there was very little evidence of facet-specific genetic influences. There was considerably stronger evidence for facet-specific environmental influences, where each facet is strongly influenced by environmental experiences that are specific to that facet. However, there was also evidence that particular environmental influences--especially those that shape disease conviction--also influence some of the other HA facets (fear and interference). The importance of environmental factors is consistent with the cognitive-behavioral model of HA, although the model needs to be refined to account for the role of genetic factors. Possibilities for refining the model are discussed, along with promising research directions to better understand the role of genes and the environment in HA.