Over 50% of the world's population still relies upon traditional biomass fuel to fulfill its energy needs. This paper examines the serious health, gender, and demographic consequences of traditional biomass fuel use. We hypothesize that exposure to indoor air pollution in the form of particulate smoke generated by burning biomass may lead to high levels of infant and child mortality from acute respiratory infections and through other mechanisms. Using national data to make crosscountry comparisons, we show that biomass fuel consumption is highly correlated with infant and child mortality rates, even after controlling for income, education, and other variables. While our analysis does not assess whether the relationship is causal, the empirical evidence suggests that traditional energy use patterns are associated with high mortality rates in developing countries that rely heavily on biomass fuels. High mortality rates may, in turn, lead to high levels of fertility because of both the "need" to replace children and risk aversion. The effect on fertility may be compounded by the utility derived from children's important labor contributions in gathering biomass fuel. Thus, the use of traditional biomass fuels may delay the demographic transition from patterns of high mortality-high fertility to patterns of low mortality-low fertility. These findings have important implications for the development of strategies to improve the quality of rural life and alleviate poverty in developing countries.