This paper evaluates the contribution of an electronic health records (EHR) system to efficient decision-making by physicians, and investigates whether these systems lead to more efficient medical care in emergency departments (ED). Log-files of patient visits and admissions were retrieved from an integrative EHR system that serves seven main hospitals owned by a large health maintenance organization (HMO). This study focused on readmissions within seven days and single-day admissions, problems that concern hospitals around the world. The findings indicate that using an EHR system in the EDs correlates with a decreased number of readmissions within seven days as well as with a reduced number of single-day admissions. The results provide evidence that using EHR system may contribute to efficiency in an ED by assisting decision-making. We believe this is the first data set that investigates the impact of an EHR on hospital efficiency at the scale of HMO.