This study examines the relationship between basic assumptions, posttraumatic growth (PTG), and ambiguity tolerance (AT). It is hypothesized that basic assumptions will mediate the relationship between AT and PTG and that trauma will have a moderating effect in this model. A group of 274 university and college students, average age of 23, who either experienced a traumatic event or not, completed the following questionnaires: Measure of Ambiguity Tolerance, World Assumptions Scale, and Posttraumatic Growth Inventory. The results indicate that the relationship between AT and PTG is mediated by basic assumptions but only among participants who were not exposed to a traumatic event. Thus, the trauma is a moderating variable in this model. The findings are discussed in terms of a defensive search for meaning following stressful life events, according to the individual’s perception of the event.