This study examined the relationship between personality disorder (PDs) and 7-month treatment outcome in 197 men admitted to methadone maintenance. Subjects reported pervasive improvement, and the amount of improvement did not significantly differ for those subjects with and without PDs. PD subjects entered treatment with more severe self-reported drug, alcohol, psychiatric, and legal problems, and despite progress, remained more problematic in those areas relative to subjects without PDs. Subjects with antisocial PD had admission and 7-month problem status similar to subjects with other PDs. The 7-month urinalysis results for opiates and cocaine showed no significant differences between subjects with and without PDs. Fewer PD subjects stayed in treatment continuously for the 7-month period. Several cluster B PDs-borderline, antisocial, and histrionic-predicted poorest overall outcomes. Methadone-maintained patients with PDs may warrant additional treatment services if they are to approach the functional level of patients without PDs.