Background: The role of genetic and environmental factors for developing alexithymia is still unclear, and the aim of this study was to examine these factors in a large population-based sample of twins. Methods: The Toronto Alexithymia Scale-20 (TAS-20) was included in a mail survey of 46,418 individuals born between 1931 and 1982 and registered with the Danish Twin Registry. The response rate was 75.3%. A total of 8,785 twin pairs, where both cotwins had completed all items of the TAS-20, were selected for this study. Analyses were conducted for total TAS-20 scores and the subscales of (1) difficulties in identifying feelings, (2) difficulties in describing feelings, and (3) externally oriented thinking. The phenotypes were analyzed both as categorical and continuous data. Results: All measures of similarity suggested that genetic factors added to all facets of alexithymia. Structural equation modeling of the noncategorical data, an ACE model including additive genetic, shared environmental and nonshared environmental effects, provided the best fit for all three facets of alexithymia as well as total alexithymia scores, with heritabilities of 30–33% and the remaining variance being explained by shared (12–20%) and nonshared environmental effects (50–56%). Conclusion: The results from this large population-based sample suggest that genetic factors have a noticeable and similar impact on all facets of alexithymia. While the results suggested a moderate influence of shared environmental factors, our results are in concordance with the general finding that environmental influences on most psychological traits are primarily of the nonshared rather than the shared type.