Although recent research suggests links between optimism and mental health, little is known about the genetic and environmental origins of these links or of optimism itself. The Life Orientation Test of optimism and pessimism and various measures of self-reported mental health (depression, life satisfaction, paranoid hostility, and cynicism) were administered to over 500 same-sex pairs of middle-aged identical and fraternal twins, half of whom were reared together and half adopted apart early in life. Twin/adoption analyses yield significant heritability estimates of about 25% for both optimism and pessimism; shared rearing environmental influence was also significant for optimism but not for pessimism. Both optimism and pessimism contributed independently to the prediction of depression and life satisfaction; pessimism but not optimism predicted paranoid hostility and cynicism. These associations diminished little when neuroticism was controlled. Multivariate genetic analyses of the multiple correlations for the mental health variables suggest that genetic factors contribute appreciably to associations between optimism/pessimism and mental health.