The primary goal of this study was to examine how perfectionism affects psychological symptoms during the course of treatment. We examined session-by-session symptom changes in a sample of 105 adult clients who presented for counseling at a psychology training clinic housed at a large Midwestern university in the United States. Using a recently developed measure of perfectionism (Short Almost Perfect Scale [SAPS]) that possesses good psychometric features, we were able to investigate effects of both maladaptive (high self-criticism) and adaptive (high standards with low self-criticism) perfectionistic characteristics on indicators of personal and interpersonal psychological distress across time. Multilevel modeling analyses indicated that both symptomatic distress and interpersonal problems improved over the course of therapy. Maladaptive perfectionism was associated with higher levels of interpersonal problems and distress at the outset of therapy, and related differentially to change patterns in symptom distress and interpersonal problems over the course of treatment. Maladaptive perfectionism, however, was not related to level of symptoms at the end of therapy. Adaptive perfectionistic characteristics were associated with fewer interpersonal problems at the beginning and end of therapy. Results suggest the value of assessing perfectionistic characteristics at the onset of treatment, even for clients not presenting with obvious concerns linked to such individual differences.