The purpose of this study was to develop and validate a self-rating scale for a differentiated assessment of mentalization. A pool of 40 items was developed and evaluated on n=434 inpatients with mental disorders at three time points. Symptom severity, self-injuring behavior, suicidal tendency and attachment style were also assessed. A varimax-rotated factor analysis supported the extraction of four factors. The model fit was checked by confirmatory factor analysis. Internal consistency of the mentalization questionnaire (MZQ) was .81. The relation between symptom severity and MZQ scores was found to be significant, and significant group differences were found between patients displaying self-injuring behavior and those who did not as well as between patients with and without suicide attempts. The MZQ can be considered as a practicable self-rated instrument with acceptable reliability and sufficient validity to assess at least aspects of mentalization in patients with mental disorders.