Evidence linking electronic health record (EHR) adoption to better care is mixed. More nuanced measures of adoption, particularly those that capture the common incremental approach of adding functions over time in US hospitals, could help elucidate the relationship between adoption and outcomes. We used data for the period 2008-13 to assess the relationship between EHR adoption and thirty-day mortality rates. We found that baseline adoption was associated with a 0.11-percentage-point higher rate per function. Over time, maturation of the baseline functions was associated with a 0.09-percentage-point reduction in mortality rate per year per function. Each new function adopted in the study period was associated with a 0.21-percentage-point reduction in mortality rate per year per function. We observed effect modification based on size and teaching status, with small and nonteaching hospitals realizing greater gains. These findings suggest that national investment in hospital EHRs should yield improvements in mortality rates, but achieving them will take time.