Perfectionism is conceptualized as having two higher-order dimensions that consists of perfectionistic strivings and perfectionistic concerns. Both are associated with different forms of mental distress and are usually assessed using such self-report measures as the Frost Multidimensional Perfectionism Scale (FMPS) and the Multidimensional Perfectionism Scale (MPS). Whether elevated levels of these dimensions could be determined by a behavioral measure has, however, received less attention, and no study has used it in a clinical trial. Hence, the current study investigated the use of a behavioral measure based on the Pennebaker’s essay writing test. In experiment I, 97 participants were recruited in a university setting and instructed to complete an essay. The participants were randomized into two conditions, primed to remember an occasion where they either failed or succeeded, followed by an opportunity to make corrections. Self-report measures of perfectionism were also completed before and after the experiment. The results indicate that the dimension perfectionistic strivings were correlated to making more changes among those in the fail-condition, rs = .30-.34, while the findings for the positive-condition were mixed. In experiment II, 102 participants receiving Internet-based Cognitive Behavior Therapy (ICBT) for perfectionism or being on a wait-list control completed the same essay writing test at pre- and post-treatment, but without being primed. The results revealed change over time with regard to making more errors for those participants assigned to ICBT, Cohen’s d = 0.27, however, there were no interaction effects between the two conditions. In addition, regression analyses between the essay writing test and self-report measures of perfectionism did not reveal any significant relationships. Overall, the usefulness of a behavioral measure for perfectionism based on an essay writing test is unclear, at least in terms of assessing treatment outcome. Future research should investigate if other types of behavior could be perceived as more personally relevant, thereby inducing more perfectionistic strivings and perfectionistic concerns.