Systematic assessment of 12 ego functions in 25 schizophrenics, 25 neurotics, and 25 normals was carried out in a 5-year research project. In the present article this ego function assessment is applied to the study of a single patient in psychoanalytic psychotherapy. Utilizing the intensive design technique of statistical study, the effect of diazepam (Valium) in combination with psychotherapy was studied. Assessments were made, by the senior author as therapist, in a double blind situation, after every one of the two weekly therapy sessions, over a 6-month period, and compared to independent ratings by the experimental interview developed in the research project.The results suggest that clinical assessment of ego functions can be made validly and reliably, and that the therapeutic effect of diazepam can be discerned over and above the effect of time and psychotherapy.The intensive design technique is likely to be useful generally for research in psychoanalysis.