This case report describes the reconstruction of a severe childhood trauma at about the age of three and a half years in a patient suffering from the symptoms of derealization. The traumatic experience was the witnessing of suicidal attempts on the part of the mother. The symptoms resulted from an identification with the 'hanging mother'. In the discussion the author suggests that the usual mechanism of repression failed and compelled the ego to revert to primitive identification, and that the transition from the pre-oedipal to the oedipal phase is associated not only with a more stable type of identification but also with the development of certain perceptual processes with the separation of phantasy from reality. In the case reported the event which led to the symptoms of derealization occurred during the transition period postulated above, with the result that a reality event became confused with phantasy.