Posttraumatic stress disorder (PTSD) is prevalent among substance use disorder (SUD) patients. Although Criterion A trauma is critical to the formulation of a PTSD diagnosis, little research has examined the reliability of retrospective reports of such stressors and factors that affect reporting among these patients. This study examined these issues among SUD patients. Participants (N = 120) were assessed by interviews and questionnaires after entering inpatient SUD treatment and at a 6-month follow-up. About 40% of participants met criteria for a current PTSD diagnosis. Results revealed moderate stability of Criterion A trauma reports, which improved when Criterion A requirements were relaxed (i.e., participants were required to report the stressor but not the Criteria A1 and A2). Intrusive symptoms were associated with increased stressor reporting over time, whereas numbing symptoms and SUD abstinence were associated with decreased stressor reporting over time. Dissociative symptoms were associated with changes in reporting in either direction.