Background Personality pathology affects behavioral patterns in patients with schizophrenia notwithstanding psychotic symptomatology. An investigation of the role of co-morbid personality pathology in the occurrence of aggression in schizophrenia is explored using both categorical and dimensional approaches to personality pathology. Methods In a cross-sectional study we evaluate, in 97 patients diagnosed with schizophrenia, the effect of personality pathology on the occurrence of aggression in schizophrenia using both a categorical approach, as described in DSM-IV-TR Axis II, and a dimensional approach, as operationalized in the Dimensional Assessment of Personality Pathology-Basic Questionnaire (DAPP-BQ). We also employ mediation analyses to explore the extent to which dimensions within the DAPP mediate the relationship between co-morbid personality disorders and aggression. Results Personality pathology accounts for aggression in schizophrenia. Both the categorical and the dimensional approaches equally well account for the occurrence of aggression, with each model accounting for 60% of the variance. Interestingly, the mediation analysis reveals that the association between categorically defined personality pathology and aggression is substantially mediated by the higher-order-trait dissocial behavior of the DAPP-BQ, accounting for 50.6 % of the total effect size. Conclusion Personality pathology can be a significant predictor of aggression in patients with schizophrenia. While both the categorical and the dimensional trait models of personality disorders equally explain the aggression data, much of the relationship between the categorically defined personality disorders and the occurrence of aggression in schizophrenia can be explained by the presence of dissocial behavior as operationalized in the DAPP-BQ dimensional model.