Attempts to follow the treatment Progress of patients with multiple personality havebeen limited to a few single case studies and, in the only large series, to those most successfully treated. This study followed the treatment progress of 20 patients for a mean of 39 months after intake. The study included a detailed history, neurological examination, and psychological testing on each patient. At follow-up each patient was interviewed and a questionnaire was completed by his or her therapist to assess treatment response. Although nine patients achieved partial or full integration, complete integration was maintained by only five. During therapy the unintegrated patients experienced emotional trauma at approximately twice the rate as did the patients who eventually became integrated. Psychodynamic psychotherapy and hypnosis were the most widely prescribed therapies. The improvement in multiple personality appeared to proceed in a stepwise fashion from acceptance of the diagnosis to integration. The progress of therapy was hindered most commonly by the overuse of the mental mechanisms of repression and denial, the continued utilization of secrecy, which began during child abuse, and the production of numerous crises. The most common countertransferences included anger, exasperation, and emotional exhaustion. Although the psychotherapy of patients with multiple personality is tedious and time consuming, it can be eminently successful if the patient and therapist persevere.