Perfectionism has previously been identified as having a significant negative impact on therapeutic outcome at termination in the brief (16-week) treatment of depression (S. J. Blatt, D. M. Quinlan, P. A. Pilkonis, & T. Shea, 1995) as measured by the 5 primary outcome measures used in the National Institute of Mental Health Treatment of Depression Collaborative Research Program (TDCRP). The present analyses of other data from the TDCRP indicated that this impact of perfectionism on therapeutic outcome was also found in ratings by therapists, independent clinical evaluators, and the patients and that this effect persisted 18 months after termination. In addition, analyses of comprehensive, independent assessments made during the treatment process indicated that perfectionism began to impede therapeutic gain in approximately 2/3 of the sample, in the latter half of treatment, between the 9th and 12th sessions. Implications of these findings are discussed, including the possibility that more perfectionistic patients may be negatively impacted by anticipation of an arbitrary, externally imposed termination date.