Patients suffering from anxiety disorders other than posttraumatic stress disorder (PTSD) interpret anxiety responses themselves as evidence that threat is impending: "if anxiety, then threat" (Arntz, Rauner, & van den Hout, 1995, Behaviour Research and Therapy, 33, 917-925). This "emotion-based reasoning" (ER) may render a disorder self-perpetuating. Analogous to ER, danger might also be inferred from the presence of intrusions: "intrusion-based reasoning" (IR). The aims of this study were to test whether ER and IR are involved in chronic PTSD. Vietnam combat veterans with or without PTSD or other anxiety disorders rated perceived danger of brief scenarios in which information about objective danger (danger vs safety) and response (anxiety/intrusions vs non-distressing emotion) was systematically varied. Two series were administered: ER-scenarios were non-specific for PTSD and IR-scenarios were specific for PTSD. Relative to control participants, PTSD patients engaged in both ER and IR: whereas veterans without PTSD inferred the danger of scenarios from objective stimulus information, veterans with PTSD also inferred danger from the presence of anxiety or intrusions. Further analyses showed that these effects were largely mediated by perceived uncontrollability.