Aim: To analyse patterns and trends in mortality from unknown causes (sudden infant death syndrome and unascertainable deaths) for all Aboriginal and non-Aboriginal infants born in Western Australia, 1980-2001.Methods: Using total population linked data, we reviewed all post-mortem reports, including death scene investigations and final causes of death as ascertained by the coroner. Neonatal, post-neonatal and infant mortality rates attributed to unknown causes were calculated and the latter were analysed according to maternal age, geographical location, gestational age, sex and birthweight. Relative risks (95% confidence interval) for Aboriginal infants (compared with non-Aboriginal) were calculated.Results: The proportion of deaths considered to be of unascertainable cause has significantly increased in recent years. In contrast to the non-Aboriginal population, there has been no significant decrease in deaths in the Aboriginal population and the overall relative risk for Aboriginal infants for the most recent years studied was 7.9 (95% confidence interval 5.1-12.2). The relative risk was significantly increased for most categories analysed.Conclusion: Reviewing post-mortem reports enabled identification of changes in the classification of deaths due to unknown causes. This provided a more accurate picture of the patterns and long-term trends of such deaths so that programmes can be developed to specifically target those groups most at risk. Adequately funded and evaluated education campaigns aimed at reducing the risk of sudden infant death syndrome among Aboriginal infants are required, as well as sustaining the current efforts that have been so successful for non-Aboriginal infants.