This paper examines the effect of waterborne lead exposure on infant mortality in American cities over the period 1900 to 1920. Variation across cities in water acidity and the types of service pipes, which together determined the extent of lead exposure, identifies the effects of lead on infant mortality. In 1900, a decline in exposure equivalent to an increase in pH from 6.675 (25th percentile) to 7.3 (50th percentile) in cities with lead-only pipes would have been associated with a decrease in infant mortality of 7% to 33%, or at least twelve fewer infant deaths per 1,000 live births.