Previous research using exploratory factor analysis of the Impact of Event Scale (Horowitz et al., 1979) has suggested structures that are not consistent with the original two-factor model. IES data from 485 emergency service personnel who had experienced traumatic events during the course of their work were submitted to confirmatory factor analysis to test seven alternative factor models of the IES. This study is one of the most comprehensive analyses of the IES to date, investigating both theoretically and empirically derived models. The model of best fit was a four-factor model with a single second-order factor. This model comprised four first-order factors including intrusion, avoidance, numbing and sleep disturbance factors all subsumed by a second-order general distress factor. These results provide support for the multidimensionality of both intrusion and avoidance symptoms following traumatic experiences. Researchers and practitioners in the field of traumatic stress should be made aware of the assessment and treatment implications of this multidimensional structure. (C) 2003 Elsevier Ltd. All rights reserved.