--- - |2+ Abstract Objectives Associations between interpersonal problems and mentalization have rarely been investigated. In this study, we explored patterns of interpersonal problems, mentalization, symptom severity, and attachment during inpatient treatment and at follow‐up. Additionally, we investigated whether mentalization predicts a decrease in interpersonal distress. Method  We analyzed time‐series data from patients with mental disorders. Data were collected at the beginning and at the end of inpatient treatment, and approximately 6 months after discharge from hospital. Results Patterns of correlations were stable from admission to the hospital until follow‐up. Treatment significantly increased the levels of mentalization and decreased the levels of interpersonal problems and symptom severity, whereas attachment was only partially targeted. Improvements in mentalization significantly predicted reduction in interpersonal distress at each point in time. Conclusion Results revealed characteristic patterns of interpersonal problems, mentalization, symptom severity, and attachment. Mentalization was found to play a key role in the reduction of interpersonal distress. - Journal of Clinical Psychology, EarlyView.