The effects of dispositional optimism, as defined by generalized positive expectations for the future, on physical health are mixed, especially in diseases that can be immunologically mediated such as HIV and cancer. Both experimental and naturalistic studies show that optimism is negatively related to measures of cellular immunity when stressors are difficult (e.g., complex, persistent, and uncontrollable) but positively related when stressors are easy (e.g., straightforward, brief, and controllable). Although the negative relationship between optimism and immunity has been attributed to the violation of optimists' positive expectancies and subsequent disappointment, empirical evidence suggests that it is more likely to be a consequence of optimists' greater engagement during difficult stressors. For example, negative mood does not account for the effect, but conscientiousness, a personality facet related to engagement, does. The mixed immunological correlates of optimism may explain why it does not consistently predict better disease outcomes.