The American Recovery and Reinvestment Act identified secure patient-physician e-mail messaging as an objective of the meaningful use of electronic health records. In our study of 35,423 people with diabetes, hypertension, or both, the use of secure patient-physician e-mail within a two-month period was associated with a statistically significant improvement in effectiveness of care as measured by the Healthcare Effectiveness Data and Information Set (HEDIS). In addition, the use of e-mail was associated with an improvement of 2.0-6.5 percentage points in performance on other HEDIS measures such as glycemic (HbA1c), cholesterol, and blood pressure screening and control.