Previous research suggests that medical advances played a negligible role in the large decline in mortality rates during the first half of the twentieth century. This paper, in contrast, presents evidence that sulfa drugs―the first pharmaceuticals effective at treating infectious diseases― were an important cause of U.S. mortality declines after their discovery in the 1930s. Using timeseries and difference-in-difference methods (with infectious diseases unaffected by sulfa drugs as a comparison group), we present evidence on the effects of sulfa drugs on mortality. We find that sulfa drugs led to a 25% decline in maternal mortality, a 13% decline in pneumonia and influenza mortality, and a 52% decline in scarlet fever mortality between 1937 and 1943. Sulfa drugs also widened racial disparities in mortality, suggesting that new medical technology diffuses more rapidly among whites than blacks and consistent with the hypothesis that innovation initially increases inequality across population subgroups.