Introduction: Hypochondriasis is a severe mental disorder of unknown etiology. Aims: To investigate the role of genetic and environmental factors in hypochondriasis. Methods: A community sample of 167 pairs of monozygotic twins and 140 pairs of dizygotic twins completed the Illness Attitude Scales. Two empirically validated methods were used to identify cases of hypochondriasis; that is, “caseness” classifications, representing clinically significant (i.e. full or subsyndromal) hypochondriasis. Biometric structural equation modeling was used to investigate the relative importance of additive genetic factors, and shared and nonshared environmental factors. Results: Hypochondriasis was highly heritable; additive genetic factors accounted for 54 to 69% of variance, depending on assessment method. Remaining variance was due to nonshared environment (i.e., environmental factors not shared by members of a given twin pair, such as illnesses experienced only by one member of a given twin pair). Effects of shared environment (factors affecting both twins of a given twin pair) accounted for no variance. Conclusion: To our knowledge, this is the first attempt to estimate the role of genetic and environmental factors for hypochondriasis (defined in terms of caseness). Results highlight the importance of genetic factors. The findings also partially support contemporary cognitive­behavioral models, which emphasize the role of maladaptive learning through environmental experiences. That is, results are consistent with the role of personal illness (e.g., childhood hospitalizations). However, the results do not support the role of parental modeling or parenting styles that lead the child to view oneself as sickly or highly vulnerable to disease (shared environmental factors).