The purpose of this study was twofold: (a) to empirically evaluate the effects of a 21-day wilderness therapy program (WT) on the defense styles, perceived psychosocial stressors (expressed concerns), dysfunctional personality patterns, clinical syndromes, and maladaptive behaviors of 109 troubled adolescents, as measured by the Defense Style Questionnaire-40, Millon Adolescent Clinical Inventory (MACI), and Youth Outcome Questionnaire-2.0 (Millon, 1997); and (b) to begin to identify the types of clinical concerns on Axes I, II, and IV for which wilderness therapy is most effective. Wilderness therapy resulted in statistically significant improvement on immature defense and maladaptive behavior scores, and on the Expressed Concerns, dysfunctional Personality Patterns, and Clinical Syndromes scores of subjects. Moderate to large effect sizes were found for a wide range of clinical concerns on Axes I, II, and IV. The most striking finding of this study is that WT appears to facilitate positive characterological change in adolescents with clinically elevated MACI Personality Patterns scores. Short-term interventions leading to characterological change are virtually unheard of in the personality literature. Future research is needed to confirm whether or not WT is effective for treating budding personality disorders.