The Narcissistic Personality Inventory (NPI) has dominated research on narcissism in the field of social and personality psychology. Surprisingly, it is unclear whether the NPI is useful for identifying pathological narcissism in patients with Narcissistic Personality Disorder (NPD). The goal of this study was to close this research gap. We used an extreme-group approach by including NPD patients and healthy controls and comparing their narcissism scores. We further investigated whether explicit self-esteem (assessed with the Rosenberg Self-Esteem Scale) suppressed the relationship between group membership and NPI narcissism. According to our results, NPD patients do not score higher on the NPI in comparison to healthy controls. Analysis of indirect effects revealed that differences in NPI scores are suppressed by NPD patients' low self-esteem. Our results indicate that the NPI is not a valid indicator of NPD, unless one controls for self-esteem. Implications for future research are discussed.