The National Vietnam Veterans Readjustment Study (NVVRS) researchers reported that 30.9% of all men who served in that conflict developed posttraumatic stress disorder (PTSD) even though only about 15% had been assigned to combat units. Scholars, mainly historians, have questioned the accuracy of the PTSD prevalence rate. The purpose of this article is to evaluate the merits of several hypotheses adduced to explain the high apparent PTSD prevalence in the NVVRS. Empirical and conceptual analysis suggests that malingering is unlikely to account for many cases. Also, deployment to Vietnam in the absence of exposure to classic traumatic stressors is likewise unlikely to account for many cases. There are three plausible explanations for the high prevalence rate of PTSD in the NVVRS. First, DSM-III-R PTSD criteria, used in the NVVRS, did not require that symptoms produce impairment. Accordingly, some PTSD-positive cases may actually have been leading productive lives, despite occasional nightmares, or other stress responses. Second, some men assigned to non-combat duty (e.g., medics) may have been exposed to PTSD-inducing stressors. Third, some respondents may have invoked the Vietnam-PTSD narrative to make sense of postwar psychological difficulties having diverse causes unrelated to their military service. These three factors likely contribute to the high PTSD prevalence rate in the NVVRS.