We investigated the validity of different subtypes of borderline personality organization (BPO) as assessed by theory-driven profiles of the Minnesota Multiphasic Personality Disorder (MMPI; Hathaway & McKinley, 1943 ) Dutch Short Form (DSFM; Eurelings-Bontekoe, Onnink, Williams, & Snellen, 2008 ) in a naturalistic follow-up study among 2,062 psychiatric outpatients who received 6 months of ambulatory treatment. Patients were assessed at intake (T1) and 6 months later (T2). At T2, both patients and therapists rated the level of improvement, using the Global Assessment of Improvement. Patients with the high-level BPO profile showed the largest increase in well-being and the largest decrease in severity of symptomatology, whereas severity of symptomatology and well-being of patients with psychotic BPO profiles did not change over time. Agreement between patients and therapists about improvement was good for the internalizing immature BPO and high-level BPO patients, but poor for the externalizing low-level BPO and narcissistic patients.