This study examined the association of Axis I and Axis II disorders among offenders who were in prison-based substance abuse treatment in a national multi-site study. Participants (N = 280) received a psychosocial assessment and a structured diagnostic interview in two separate sessions. Logistic regression models examined the association between lifetime mood and anxiety disorders with two personality disorders, and the relationship of Axis I and Axis II disorders (alone and in combination) to pre-treatment psychosocial functioning. Over two-thirds of the sample met criteria for at least one mental disorder. Borderline personality disorder was strongly associated with having a lifetime mood disorder (odds ratio = 7.5) or lifetime anxiety disorder (odds ratio = 8.7). Individuals with only an Axis II disorder, or who had both Axis I and Axis II disorders, had more severe problems in psychosocial functioning than those without any disorder. Clinical treatment approaches need to address this heterogeneity in diagnostic profiles, symptom severity, and psychosocial functioning. Copyright © 2008 John Wiley & Sons, Ltd.