Fairbairn's interest in DID was central to the development of his model of endopsychic structure. We describe Dr Finnegan's clinical work with Evelyn, a woman with DID. We outline a recent expansion of Fairbairn's object relations theory by Dr Clarke reconciling it with Freud's topographic theory. We retrospectively apply this Fairbairnian model to Dr Finnegan's clinical findings. Alter personalities are understood in relation to Fairbairn's dynamic structures and related processes of dissociative splitting. We argue that the dissociations characteristic of DID function to both preserve and obscure the object relations at their origin and we account for the resistances found in the therapeutic process by reference to Fairbairn's concept of attachment to internalised bad objects. We discuss our findings relative to works by Davies and Frawley, and by Grotstein.