Optimism has been shown to be important in maintaining wellbeing into old age, but little is known about the sources of variation in optimism and its links to mental and somatic health. Optimism, mental, and self-rated health were measured in 3,053 twin individuals (501 MZF, 153 MZM, 274 DZF, 77 DZM, and 242 DZ opposite-sex twin pairs and 561 single twins) over 50 years using the life orientation test, the General Health Questionnaire and a single-item question for self-rated health. Additive genetic factors explained 36, 34, and 46% of the variation in optimism, mental, and self-rated health, respectively, with the remainder being due to non-shared environmental influences. Genetic influences accounted for most of the covariance between the variables (14–20% of the genetic variance) indicating that in older adults genes predisposing to high optimism also predispose to good mental health and self-rated health.