Background: Electronic messaging (EM) is increasingly utilized among patients with diabetes, but it is unclear whether it is associated with improved glycemic control, hospital admissions, or emergency visits. Methods: Patients who were seen over a 1 year period at an academic endocrinology clinic with a diagnosis of diabetes were categorized according to portal activation and whether EMs were actually sent. The association between EM and HbA1c and inpatient or emergency department (ED) visits was further characterized using multivariable (MV) linear or logistic regression models. Results: A total of 867 patients were using EM (active user group), 1207 patients had activated the patient portal but did not use the EM function (active nonuser group), and 1542 patients had not activated the patient portal (inactive group). There were important disparities in race, age, income, and other variables across groups. The HbA1c was 7.7% -1.5%, 8.4% -1.9%, and 8.2% -1.8% among the active user, active nonuser, and inactive groups, respectively (P < 0.0001). After controlling for other factors, EM was associated with a 0.25% (SE 0.04%) lower HbA1c compared with the active nonuser group and a 0.19% (SE 0.04%) lower HbA1c compared with the inactive group (P < 0.0001 for both). However, EM frequency was not associated with HbA1c. EM use was not associated with inpatient or ED visits in MV analysis. Conclusions: EM use was associated with improved glycemic control, even after controlling for electronic portal access and other variables, but not with hospitalizations or emergency visits. EM frequency was not associated with glycemic control, justifying the need for prospective studies.