A case-control study of mortality from acute respiratory infections (ARI) among children under five years of age was conducted in Naucalpan, an urban-suburban area of Mexico City, and in rural localities of Tlaxcala, Mexico. The study found that ARI deaths tended to occur in the poorest neighborhoods; 78% of the deceased study subjects were infants under six months old; and 68% of the deaths occurred at home. Comparison of the data for cases (fatalities) and control children who had severe ARI but recovered showed that failure to receive antibiotics was associated with death (odds ratio 28.5, 95% confidence interval 2.1-393.4). This antibiotic effect was controlled for numerous potentially confounding factors. It is evident that antibiotics had a much greater effect in the early days of the illness than later on. In general, the findings strongly support PAHO/WHO primary health care strategies--including such strategies as standardized management of severe ARI cases--that seek to reduce childhood ARI mortality.