Clinical accounts suggest grandiosity is an important characteristic of both psychopathy and narcissism. This study examined the relationships between grandiosity and each of these personality disorder syndromes, using a novel index, namely the Grandiosity Index. Seventy-five incarcerated males completed clinical interviews of psychopathy and narcissism and a self-report inventory of narcissism and were rated on interpersonal measures of psychopathy and narcissism. Trained research assistants rated participants on the Grandiosity Index. Analyses provided preliminary evidence of construct validity for the index. Scores on the Grandiosity Index were significantly correlated with scores on both clinical and interpersonal measures of psychopathy and narcissism and with self-reported narcissism. Regressions demonstrated that the Grandiosity Index explained substantial unique variance in psychopathy after controlling for scores on narcissism. This study demonstrates that grandiose features are associated uniquely with clinical ratings of psychopathy, though not significantly more with psychopathy than with narcissism.