Aim: The present study aims to estimate the relative importance of genetic and environmental factors for health-related quality of life (HRQL) measured by the 12-item Short-Form Health Survey (SF-12). Methods: The study was based on two Danish twin cohorts (46,417 twin individuals) originating from the nationwide, population-based Danish Twin Registry. The twins were approached by a mailed-out questionnaire in 2002. The questionnaire included the SF-12, information on demographic factors, and questions on a variety of specific diseases. Heritability of the SF-12 includes the physical component summary (PCS) and the mental component summary (MCS); and etiologically important variance components were estimated using multivariate biometric models. The respondents were stratified into six groups, based on age and sex. Results: A total of 33,794 (73%) individual twins responded to the survey. The SF-12 was completed by 29,619 individuals, which included 9,120 complete twin pairs. Overall, the best-fitting model explaining the variance of HRQL was the ACE model. The estimated heritability of the SF-12 was between 11% and 35%, whereas between 65% and 89% could be explained by unique environmental or stochastic factors in the different sex and age groups. The highest heritability was seen among older twins. In addition, the genetic correlation between MCS and PCS scores was low (0.07 and 0.23 for males and females, respectively) among younger and high (0.26 and 0.45 for males and females, respectively) in the oldest age group. Both the largest genetic influence on HRQL and the largest genetic overlap between the scores were seen in the oldest age group, which consisted of twins older than 55. The unique environmental correlation between MCS and PCS were generally negative. Conclusion: The heritability of HRQL differs between different age groups. In general, most of the variance in the SF-12 summary components was determined by unique environmental factors.