Background: Although rumination is a key process in the onset and maintenance of depressive symptoms and a powerful predictor of persistent posttraumatic stress disorder (PTSD), little is known about the differences and similarities of rumination in these conditions. Previous research has not always differentiated between rumination and intrusive images. Aims: We sought to systematically evaluate rumination and to gather more information about the content and associated emotions in three patient groups (PTSD, and depressed with and without trauma; n = 65). Furthermore, we examined the interaction between rumination and another predominant intrusive cognition, intrusive image. Method: A multi-method assessment for rumination, including a rumination questionnaire and a rumination log (kept for one week), was employed. Results: Rumination was found to be complex and composed of subcomponents that are similar across the diagnostic groups. Rumination rarely stopped intrusive images and it made the participants feel worse. There were, however, also important differences: in PTSD, rumination always or often triggered intrusive images and the traumatized individuals (PTSD and depressed with trauma) ruminated more than non-traumatized depressed patients. Conclusions: The results corroborate the assumption of rumination being a transdiagnostic process, with similarities but also with important differences across diagnostic groups. Moreover, the findings support recent research on the intricate relationship between different types of intrusive cognitions.