BACKGROUND: WHO estimates that, in 2015, nearly 1 million children younger than 5 years died from lower respiratory tract infections (LRIs). Ambient air pollution has a major impact on mortality from LRIs, especially in combination with undernutrition and inadequate health care. We aimed to estimate mortality due to ambient air pollution in 2015, particularly in children younger than 5 years, to investigate to what extent exposure to this risk factor affects life expectancy in different parts of the world. METHODS: Applying results from a recent atmospheric chemistry-general circulation model and health statistics from the WHO Global Health Observatory, combined in integrated exposure-response functions, we updated our estimates of mortality from ambient (outdoor) air pollution. We estimated excess deaths attributable to air pollution by disease category and age group, particularly those due to ambient air pollution-induced LRIs (AAP-LRIs) in childhood. Estimates are presented as excess mortality attributable to ambient air pollution and years of life lost (YLLs). To study recent developments, we calculated our estimates for the years 2010 and 2015. FINDINGS: Overall, 4·55 million deaths (95% CI 3·41 million to 5·56 million) were attributable to air pollution in 2015, of which 727 000 deaths (573 000-865 000) were due to AAP-LRIs. We estimated that AAP-LRIs caused about 237 000 (192 000-277 000) excess child deaths in 2015. Although childhood AAP-LRIs contributed about 5% of air pollution-attributable deaths worldwide, they accounted for 18% of losses in life expectancy, equivalent to 21·5 million (17 million to 25 million) of the total 122 million YLLs due to ambient air pollution in 2015. The mortality rate from ambient air pollution was highest in Asia, whereas the per capita YLLs were highest in Africa. We estimated that in sub-Saharan Africa, ambient air pollution reduces the average life expectancy of children by 4-5 years. In Asia, all-age mortality increased by about 10% between 2010 and 2015, whereas childhood mortality from AAP-LRIs declined by nearly 30% in the same period. INTERPRETATION: Most child deaths due to AAP-LRIs occur in low-income countries in Africa and Asia. A three-pronged strategy is needed to reduce the health effects of ambient air pollution in children: aggressive reduction of air pollution levels, improvements in nutrition, and enhanced treatment of air pollution-related health outcomes. FUNDING: None.