Posttraumatic growth (PTG), the phenomenon of self-reported positive outcomes of trauma, is assumed to consist of two sides: a constructive and an illusory side. This study investigates the relationship between PTG and its possible illusory and constructive predictors, as well as the moderating role of posttraumatic stress disorder (PTSD) severity. One-hundred two motor vehicle accident (MVA) survivors with full, subsyndromal, and without PTSD were assessed by multiple psychometric measures targeting PTSD severity, posttraumatic growth, optimism, and openness to experience. Hierarchical regression analysis yielded differential interaction effects between PTSD severity and optimism, as well as openness facets pointing to the moderating role of PTSD severity in the prediction of an illusory and a constructive factor in PTG. © 2008 Wiley Periodicals, Inc. J Clin Psychol. 64: 245–263, 2008.