Previous analyses of data from the National Institute of Mental Health (NIMH)-sponsored Treatment of Depression Collaborative Research Project (TDCRP) revealed that patients' perfectionism had an adverse effect on treatment outcome (Blatt, Quinlan, Pilkonis, & Shea, 1995). Part of this adverse effect was accounted for by perfectionistic patients' difficulties in relating constructively to their therapist (Zurof et al., 2000). However, even after taking into account this indirect effect, the remaining direct effect of perfectionism on outcome was statistically significant, suggesting the presence of other mediators. The present study identifies one of these other mediators: the quality of the patients' social network. Specifically, patients' pretreatment perfectionism predicted a less positive social network over time, which in turn predicted less reduction of symptoms at termination. The two mediating effects that involved the social network and the therapeutic alliance accounted for the statistically significant direct effect of pretreatment perfectionism on outcome. Results highlight the impact of personality on interpersonal processes in determining the outcome of brief treatment for depression.