The assumption of population homogeneity in the measurement of dispositional optimism was examined. U.S. and Canadian respondents (N = 591) completed an optimism scale. Possible population heterogeneity was analyzed using factor mixture modeling. Two major results emerged. First, population homogeneity was not supported: A large class of participants had trouble giving consistent answers to optimism items, optimism and pessimism items, or, especially, pessimism items. Second, after the removal of this problematic class, the correlation between optimism and pessimism was found to be.94, a magnitude large enough to support the assumption of unidimensionality. Although psychometric problems with the measurement of optimism have not been identified previously, this study suggests that the typical measurement of dispositional optimism requires substantial revision. The findings showcase the importance of factor mixture modeling in evaluating the psychometric properties of a measurement scale.