OBJECTIVE To investigate temporal trends in admission rates for acute lower respiratory infections (ALRI) in a total population birth cohort of non-Aboriginal and Aboriginal children. METHODS Retrospective analysis of linked population-based data using the Western Australian Data Linkage System. All singleton live births in Western Australia between 1990 and 2000 were included. Hospital admission rates per 1000 live births for ALRI before age 2 years and linear time trends for ALRI admission rates were investigated. RESULTS ALRI admission rates were 7.5 (95% confidence interval (CI) 7.2-7.7) times higher in Aboriginal than non-Aboriginal children (337 vs. 45 per 1000 live births); pneumonia rates were 13.5 (95% CI 12.8-14.4) times higher and bronchiolitis rates were 5.8 (95% CI 5.3-6.0) times higher. ALRI admission rates rose in non-Aboriginal children (<12 months, 6%/year, P<0.002; 12-23 months, 11%/year, P<0.001) but declined in Aboriginal children aged 12-23 months (4%/year, P=0.003). Bronchiolitis rates rose in all children, especially non-Aboriginal infants aged <12 months (13%/year, P<0.001), while pneumonia rates rose in non-Aboriginal children but declined in Aboriginal children. Declines in bronchitis and asthma were also noted. CONCLUSIONS There has been an increase in incidence of bronchiolitis before age 12 months. For children aged 12-23 months a diagnostic shift from asthma and bronchitis to bronchiolitis and changes in health service utilisation are likely explanations for diverging temporal trends. The continuing disparity between Aboriginal and non-Aboriginal children needs to be addressed and appropriate preventative measures for ALRI, and in particular bronchiolitis, are urgently needed.