OBJECTIVE: First-line treatments for posttraumatic stress disorder (PTSD) are often implemented twice per week in efficacy trials. However, there is considerable variability in the frequency of treatment sessions (e.g., once per week or twice per week) in clinical practice. Moreover, clients often cancel or reschedule treatment sessions, leading to even greater variability in treatment session timing. The goal of the current study is to investigate the impact of PTSD treatment session frequency on treatment outcome. METHOD: One hundred thirty-six women (Mage = 32.16 [SD = 9.90]) with PTSD were randomized to receive cognitive processing therapy or prolonged exposure. PTSD symptom outcome was measured using the Clinician-Administered PTSD Scale, and session frequency and consistency were measured using dates of treatment session attendance. Session frequency was operationalized using average days between session, and consistency was defined by the standard deviation of the number of days between treatment sessions. RESULTS: Piecewise growth curve modeling revealed that higher average days between sessions was associated with significantly smaller PTSD symptom reduction, with more frequent sessions yielding greater PTSD symptom reduction (p < .001, d = .82). Higher consistency was also associated with significantly greater PTSD symptom reduction (p < .01, d = .48). The substantially larger effect size for frequency suggests that average days between treatment sessions impacts treatment outcome more than consistency. Follow-up analyses revealed a longer time interval between Sessions 4 and 5 was associated with smaller reductions in PTSD treatment outcome. CONCLUSIONS: More frequent scheduling of sessions may maximize PTSD treatment outcomes. (PsycINFO Database Record