Since at least the late 1990s, HIV has been viewed as a major threat to efforts by countries to reduce under-5 mortality. Previous work has documented increased under-5 mortality due to HIV from 1990 to 1999 in Africa. The current analysis presents estimates and trends in under-5 mortality due to HIV in low- and middle-income countries by region up to 2009.The analyses are based on the national models of HIV and AIDS produced by country teams in coordination with UNAIDS and its partners for the years 1990-2009. These models produce a time series of estimates of HIV-related mortality as well as overall mortality in children aged <5 years.These analyses indicate that, in 2009, HIV accounted for roughly 2.1% (1.2-3.0%) of under-5 deaths in low- and middle-income countries and 3.6% (2.0-5.0%) in sub-Saharan Africa. The percentage of under-5 deaths due to HIV has been falling in the last decade--for example, from 2.6% (1.6-3.5%) in 2000 to 2.1% (1.2-3.0%) in 2009 in low- and middle-income countries and from 5.4% (3.3-7.3%) in 2000 to 3.6% (2.0-5.0%) in 2009 in sub-Saharan Africa. This fall in the percentage of under-5 deaths due to HIV has been driven by a combination of factors including scale-up of prevention of mother-to-child transmission programmes and treatment for pregnant women and children, as well as a decrease in the prevalence of HIV among pregnant women.