Background Axis I comorbidity complicates diagnosing axis II personality disorders (PDs). PDs might influence Axis I outcome. No research has examined psychotherapy effects on PDs of treating Axis I comorbidity. Secondary analysis of a randomized controlled trial examined PD diagnostic stability after brief psychotherapy of chronic posttraumatic stress disorder (PTSD). Methods Patients with chronic PTSD were randomly assigned to 14 weeks of prolonged exposure, interpersonal psychotherapy, or relaxation therapy. Assessments included the Structured Clinical Interview for DSM‐IV, Patient Version (SCID‐P) and Structured Clinical Interview for DSM‐IV Axis II Disorders (SCID‐II) at baseline, week 14, and for treatment responders (≥30% clinician‐administered PTSD scale improvement, defined a priori) at week 26 follow‐up. We hypothesized patients whose PTSD improved would retain fewer baseline PD diagnoses posttreatment, particularly with personality traits PTSD mimics, e.g. paranoid and avoidant. Results Forty‐seven (47%) of 99 SCID‐II patients evaluated at baseline received a SCID‐II diagnosis: paranoid (28%), obsessive–compulsive (27%), and avoidant (23%) PDs were most prevalent. Among 78 patients who repeated SCID‐II evaluations posttreatment, 45% (N = 35) had baseline PD diagnoses, of which 43% (N = 15/35) lost at week 14. Three (7%) patients without baseline PDs acquired diagnoses at week 14; 10 others shifted diagnoses. Treatment modality and PTSD response were unrelated to PD improvement. Of treatment responders reevaluated at follow‐up (N = 44), 56% with any baseline Axis II diagnosis had none at week 26. Conclusion This first evaluation of Axis I psychotherapy effects on personality disorder stability found that acutely treating a chronic state decreased apparent trait—across most PDs observed. These exploratory findings suggest personality diagnoses may have limited prognostic meaning in treating chronic PTSD.