This study investigated the stability of defensive functioning over the course of a 4-session Brief Psychodynamic Investigation (BPI). The sample included 61 outpatients from the Adult Psychiatry Department of the University of Lausanne, Switzerland. Defenses were measured from session transcripts using the Defense Manual Rating Scale (DMRS) quantitative method. Specific changes in defenses were found over the course of the ultrabrief investigation. First, the overall defensive functioning (ODF) score and the proportion of obsessional level defenses increased significantly, with a significant increase in intellectualization. Second, the number of defenses used and the proportion of narcissistic level defenses decreased, with a decreasing prevalence of devaluation and idealization. Third, high adaptive (mature) level defenses increased then decreased over the course of BPI, returning to their level at intake by the 4th session. Relief from distress and attending to the tasks of BPI tends to improve defensive functioning, but more likely returns it to usual levels rather than producing permanent change. Future studies will need to use designs that allow estimation of state changes while taking sufficient measurements to estimate potential changes in trait levels of defenses. (PsycINFO Database Record (c) 2004 APA, all rights reserved)