After the Madrid March 11, 2004 terrorist attacks, the interplay of politicians, journalists and academicians created an atmosphere of collective trauma. The authors analysed data related to these attacks in a sample of the population of Madrid (N = 503) 18-25 days after the attacks. Post traumatic stress disorder (PTSD) was systematically assessed on the basis of a self-administered interview. The data, however, shows that there is no scientific evidence at all for collective traumatization, or an epidemic of PTSD. The incidence of PTSD ranged from what can be expected as a normal prevalence in general population in Spain under non-traumatic conditions to values that, when applied to the general population, could be considered a dramatic epidemic of PTSD. These results demonstrate that inferences about the impact of traumatic events on the general population largely depend on the measure, definition and criteria used by the researcher. Slightly changing the criteria for PTSD makes an enormous difference to the amount of traumatization that is found. This may help to explain divergent and conflicting messages coming from the so-called population-based epidemiological studies on PTSD. The implications for public health policies related to collective traumatic events are discussed in relation to these results.