The predictive power of the externalizing and internalizing factors of the Child Behavior Checklist (CBCL) for mental health treatment outcomes was studied. Data were collected over 3 years at a community mental health agency. From the population of intakes at the agency, 789 cases were selected as the unrestricted sample, and 374 of these were further analyzed as a restricted sample. Parents of these children completed the CBCL and each child was assessed by a clinician-assessor. In general, the theoretical construct that was tested was whether a specific factor of personality, extroversion or introversion, as represented by the CBCL's second order factors Externalizing and Internalizing, would manifest itself in a way that would give information about mental health treatment outcomes. The study indicated that externalizng/internalizing did relate to therapist-assessed gains after a period of treatment. Internalizers were judged to make significantly greater gains in mental health treatment than externalizers. There was not a significant relationship, however, in these samples between extroversion and introversion and two other outcomes hypothesized to relate to client personality factors. These outcomes were number of treatment sessions and therapist-assessed satisfactory treatment outcome versus abrupt or indeterminate termination. Thirty-seven appendixes present a global assessment of functioning continuum, the CBCL, coding entries, and tables of results for each hypothesis tested. (Contains 19 tables, 34 appendix tables, 2 figures, and 112 references.) (SLD)