BackgroundPosttraumatic stress disorder (PTSD) and major depressive disorder (MDD) are highly comorbid (Elhai et al., 2008. J. Clin. Psychiatry, 69, (4), 597–602). Rumination is a cognitive mechanism found to exacerbate and maintain both PTSD and MDD (Elwood et al., 2009. Clin. Psychol. Rev. 29, (1), 87–100; Olatunji et al., 2013. Clin. Psychol.: Sci. Pract. 20, (3), 225–257).AimsAssess whether four rumination subtypes moderate the relationship between comorbid PTSD and MDD symptoms.MethodWe consecutively sampled patients (N=45) presenting to a mental health clinic using self-report measures of PTSD and MDD symptoms, and rumination in a cross-sectional design.ResultsRepetitive rumination moderates the relationship between PTSD and MDD symptoms at one standard deviation above the mean (β=.044, p=.016), while anticipatory rumination moderates the relationship between PTSD and MDD symptoms at mean levels and higher levels of anticipatory rumination (mean β=.030, p=.042; higher β=.060, p=.008).DiscussionRepetitive and anticipatory rumination should be assessed in the context of comorbid PTSD and MDD and interventions should focus on reducing these rumination subtypes.LimitationsResults should be replicated with other trauma populations because the number and complexity of traumatic events may impact the assessed symptoms. Constructs should also be assessed longitudinally, in order to establish causality. We are unable to confirm why rumination styles moderated the relationship between PTSD and depression or why counterfactual thinking and problem-focused thinking did not moderate the relationship between the two constructs.