Background Since there are known disparities between Aboriginal and non-Aboriginal populations in Australia, trends in infant mortality rates can be used to assess the effectiveness of programmes designed to improve the health of Aboriginal populations. We have examined mortality rates in these populations in Western Australia.Method We used the most comprehensive and valid total population data available for an Australian state to determine all-cause and cause-specific mortality for Aboriginal and non-Aboriginal infants born in Western Australia from 1980 to 2001.Findings Overall, infant mortality rates fell in both populations, but less so in Aboriginal (from 25.0 in 1980-84 to 16.1 in 1998-2001) than in non-Aboriginal infants (from 8.4 in 1980-84 to 3.7 in 1998-2001) such that disparities between the two groups increased for all major causes of infant death. The relative risk for Aboriginal compared with non-Aboriginal infants rose from 3.0 (95% CI 2.5-3.6) to 4.4 (3.5-5.5), and there were significantly more potentially preventable deaths, such as those caused by infections (5.9 per 1000 livebirths vs 0.7 per 1000 livebirths, RR 8.5, 95% CI 7.1-10.2). Additionally, for Aboriginal infants, postneonatal mortality rates were higher than neonatal mortality rates (11.2 per 1000 livebirths vs 9.7 per 1000 livebirths), trend analyses showed that previous reductions in deaths due to preterm. birth (4.3 per 1000 livebirths-1.4 per 1000 livebirths from 1980-97) were not sustained in the most recent years studied (3.5 per 1000 livebirths), and rates of sudden infant death syndrome did not fall significantly (4.9 per 1000 livebirths vs 4.7 per 1000 livebirths).Interpretation These increasing disparities between Aboriginal and non-Aboriginal infants, especially in remote areas, demand immediate action in partnership with Aboriginal communities, focusing on both access to primary health care and better living conditions. Implementation and assessment of policies to reduce the continuing social and economic disadvantage faced by Aboriginal families are vital.