While countries with higher levels of human resources for health typically have better population health, the evidence that increases in the level of human resources for health leads to improvements in population health is limited. We use a dynamic regression model to obtain estimates of both the short-run and long-term effects of changes in physicians per capita, our measure of health system resources, on infant mortality. Using a dataset of 99 countries at 5-year intervals from 1960-2000, we estimate that increasing the number of physicians by one per 1000 population (roughly a doubling of current levels of provision) decreases the infant mortality rate by 15% within 5 years and by 45% in the long-run with half the long-run gain being achieved in 15 years. We conclude that the long-run effects of heath system resources are substantially larger than previously estimated. Our results suggest, however, that countries that have delayed action on the Millennium Development Goal of reducing infant and child mortality rate by two-thirds by 2015 (relative to 1990) may have difficulty meeting this goal even if they rapidly increase resources now.