Background: Both hereditary and environmental factors are implicated in the aetiology of cutaneous neoplasms. Studies of twins make it possible to estimate the contribution of inherited genes to the development of disease.
Objective: To assess the importance of hereditary and environmental factors (including physical environment and lifestyles) in malignant melanoma and malignant nonmelanoma of the skin.
Methods: The Finnish Twin Cohort, comprising 25 882 adult like-sexed twins with established zygosity, was linked with the Finnish Cancer Registry to identify malignant skin cancers in a prospective follow-up from 1976 to 1997. Standardized incidence ratios were computed based on national rates.
Results: Sixty twins were diagnosed with melanoma and 49 twins with nonmelanoma during the follow-up. The risks of these cancers did not differ from the risk in the population at large. There was only one pair where both twins had a malignant skin cancer (dizygotic male twins both with squamous cell carcinoma).
Conclusions: The near-total lack of concordance for skin cancer in twin pairs suggests that environmental and not hereditary effects are most important in the causation of malignant skin cancers in a white population with low levels of sun exposure.