One difficulty plaguing research on dispositional optimism and health is whether optimism and pessimism are bipolar opposites or constitute distinct constructs. The present study examined the Life Orientation Test to determine whether the two-factor structure is explained by method bias (due to measurement) or substantive differences. The authors compared three measurement models: bipolar, bivariate, and method artifact. Optimism and pessimism emerged as distinct constructs due to substantive differences. The authors also considered the validity of optimism and pessimism, examining their relations with psychological and physical health outcomes. Optimism and pessimism were more similar in relation to psychological health than to other health-related behavior or physical health outcomes. However, a strongly interpretable pattern for the relation of optimism and pessimism to the health outcomes did not emerge. Further research may benefit from considering optimism and pessimism as bivariate and also should consider the conceptual components and behavioral mechanisms specific to each variable.