In this study, we examined the relationship between treatment outcome and variables from the Inventory of Interpersonal Problems Circumplex scales (IIP-C; Horowitz, Alden, Wiggins, & Pincus, 2000) in the Pennsylvania Psychological Association's Practice Research Network (PRN; Borkovec, Echemendia, Ragusea, & Ruiz, 2001). The PRN was a naturalistic observation treatment outcome study conducted with clinicians who were providing outpatient therapy. Assessment instruments, including the Compass Assessment System (Howard, Brill, Lueger, O'Mahoney, & Grissom, 1993; Sperry, Brill, Howard, & Grissom, 1996) and the IIP-C, were used to assess outcome at the 7th session (N=73) and at termination (N=42). Significant associations were identified between seventh-session outcome and most of the IIP variables. Only IIP elevation and amplitude were related to termination outcome. Elevation, amplitude, and hostile submissive problems were related to treatment length. Ad hoc analyses indicated that the IIP elevation fully mediated the relationships between interpersonal problems and seventh-session outcome but not the relationship between amplitude and outcome. We discuss the results in relation to the external validity of the IIP.