Background Homicides by mentally ill persons have led to political concerns about deinstitutionalisation. Aims To provide accurate information about the contribution of mental illness to homicide rates. Method Retrospective study of homicide in New Zealand from 1970 to 2000, using data from government sources. 'Mentally abnormal homicide' perpetrators were defined as those found unfit to stand trial, not guilty by reason of insanity, convicted and sentenced to psychiatric committal, or convicted of infanticide. Group and time trends were analysed. Results Mentally abnormal homicides constituted 8.7% of the 1498 homicides. The annual rate of such homicides was 1.3 per million population, static over the period. Total homicides increased by over 6% per year from 1970 to 1990, then declined from 1990 to 2000. The percentage of all homicides committed by the mentally abnormal group fell from 19.5% in 1970 to 5.0% in 2000. Ten per cent of perpetrators had been admitted to hospital during the month before the offence; 28.6% had had no prior contact with mental health services. Victims were most commonly known to the perpetrator (74%). Conclusions Deinstitutionalisation appears not to be associated with an increased risk of homicide by people who are mentally ill. Declaration of interest None. Funding detailed in Acknowledgements.