Michigan's recently revised Lead and Copper Rule requires water utilities to inventory existing water service lines by 2025 and replace all lead-containing lines by 2041. This article summarizes a cost-benefit analysis using new inventory data on the number of lead service lines in the state, the projected cost of their replacement, and the estimated lifetime benefits from reduced lead exposure. Replacing 423,479 lead service lines would reduce lead exposure for 420,800 newborns and result in $3.24 billion in future benefits (compared with replacement costs of $1.33 billion). This would generate net savings of $1.91 billion and a societal return on investment of $2.44 per dollar invested. These estimates are conservative and include only quantified benefits for newborn children in Michigan for the period 2020-60. More than 153,100 of the children benefiting would be non-White (of whom 78,400 would be Black or African American), and 106,900 would be in households with incomes below the federal poverty level. Sensitivity analyses show that accelerating the replacement pace would increase the societal return on investment. This primary prevention-driven policy has the potential to reduce childhood lead exposure and improve health equity.