Background The National Confidential Inquiry into Suicide and Homicide by People with Mental Illness has been collecting detailed clinical data since 1996 on a national sample of people who commit homicide, including psychiatric reports prepared for court. From 1996-2006, the Inquiry was notified of 5808 homicides in England and Wales. A diagnosis of personality disorder was made in 16% (406) of cases in psychiatric reports prepared for court. Given prevalence figures of 50-90% for personality disorder in the offender population in general, it seems likely that this is an underestimation in this population.Aims Estimate the prevalence of personality disorder in a national case series of homicide perpetrators with court reports. Investigate any variables associated with the diagnosis of personality disorder in court reports, and with specific dimensions of personality disorder. Explore potential reasons for the lack of attribution of a personality disorder diagnosis in reports. Method 600 court reports were analysed using the PAS-DOC, a document derived version of the Personality Assessment Schedule. Those with a diagnosis of personality disorder in reports were compared with those without on a number of sociodemographic, clinical, and criminological variables Focus groups and semi structured interviews were conducted with Forensic Psychiatrists with a range of experience to explore attitudes towards personality disorder.Results The prevalence of personality disorder in this sample was 56.3% (95% CI 52.3% - 60.3%). Perpetrators with previous violent offences and substance misuse were more likely to be diagnosed with personality disorder by report writers. Severe personality disorder was significantly associated with prior convictions for any violent offences and with a stranger as a victim. Complex personality disorder was associated with a family or spouse as a victim, and negatively associated with a stranger as a victim. A number of themes emerged in the focus groups and semi-structured interviews to explain the discrepancy between the identified prevalence of personality disorder and its diagnosis made by report writers. These included issues surrounding classification, comorbid mental illness, ethical issues regarding court, recommendations for verdict and disposal, treatability, service provision, training and stigma. Conclusions Personality disorder is underdiagnosed in psychiatric reports prepared for court. Reasons for this and the implications from both a clinical and ethical perspective are discussed.