BACKGROUND: Fine particulate matter (PM(2.5)) produced by landscape fires is thought to be more toxic than that from non-fire sources. However, the effects of "fire-sourced" PM(2.5) on acute respiratory infection (ARI) are unknown. METHODS: We combined Demographic and Health Survey (DHS) data from 48 countries with gridded global estimates of PM(2.5) concentrations from 2003 to 2014. The proportions of fire-sourced PM(2.5) were assessed by a chemical transport model using a variety of PM(2.5) source data. We tested for associations between ARI and short-term exposure to fire- and "non-fire-sourced" PM(2.5) using a bidirectional case-crossover analysis. The robustness and homogeneity of the associations were examined by sensitivity analyses. We also established a nonlinear exposure-response relationship between fire- and non-fire-sourced PM(2.5) and ARI using a two-dimensional spline function. RESULTS: The study included 36,432 children under 5 years who reported ARI symptoms. Each 1 µg/m(3) increment of fire-sourced PM(2.5) was associated with a 3.2 % (95 % confidence interval [CI] 0.2, 6.2) increment in the risk of ARI. This effect was comparable to that of each ∼5 µg/m(3) increment in PM(2.5) from non-fire sources (3.1 %; 95 % CI 2.4, 3.7). The association between ARI and total PM(2.5) concentration was significantly mediated by the proportion of fire-sourced particles. Nonlinear analysis showed that the risk of ARI was increased by both fire- and non-fire-sourced PM(2.5), but especially by the former. CONCLUSIONS: PM(2.5) produced by landscape fire was more strongly associated to ARI among children under 5 years than that from non-fire sources.