"Dispositional optimism, a generalized expectation for positive outcomes, also has been implicated as a factor that may protect against heart disease. However, recent research has complicated this picture. It turns out that optimism and pessimism are not necessarily polar opposites, as high optimism does not necessarily imply low pessimism, and vice-versa. There is some indication that high levels of pessimism, rather than low levels of optimism, may be a more important predictor of negative health outcomes."
An interesting comment by Richard Contrada PhD. in "How to Benefit From Research on Type A Behavior"