Although now there is accumulating research on the effectiveness of psychotherapy for perfectionism, this research has been based almost exclusively on self-report data. In this article, we describe analyses from the University of British Columbia Perfectionism Treatment Study assessing close other informant ratings of change in perfectionism traits and perfectionistic self-presentation. A total of 61 close other informants of patients who participated in a 10-week dynamic-relational treatment for perfectionism completed measures of patient trait and self-presentational facets of perfectionism at pretreatment, at posttreatment, and at a 4-month follow-up. In support of the effectiveness of the treatment, we found that close other measures of patients' self-oriented perfectionism, other-oriented perfectionism, and all three facets of perfectionistic self-presentation were significantly reduced at posttreatment and follow-up. Close other measures of patients' socially prescribed perfectionism did not show change over the course of treatment and follow-up. The findings are discussed in terms of the effectiveness of the dynamic-relational treatment of perfectionism and the utility of extending research by including close other measures of change in treatment-outcome research. (PsycInfo Database Record (c) 2020 APA, all rights reserved).