BACKGROUND: Most patients would like to be able to exchange electronic messages with personal physicians. Few patients and providers are exchanging electronic communications. OBJECTIVE: To evaluate patient characteristics associated with the use of secure electronic messaging between patients and health care providers. DESIGN, SETTING, AND PATIENTS: Cross-sectional cohort study of enrollees over 18 years of age who were enrolled in an integrated delivery system in 2005. MEASUREMENTS AND MAIN RESULTS: Among eligible enrollees, 14% (25,075) exchanged one or more secure messages with a primary or specialty care provider between January 1, 2004 and March 31, 2005. Higher secure messaging use by enrollees was associated with female gender (OR, 1.15; 95% CI, 1.10-1.19), greater overall morbidity (OR, 5.64; 95% CI, 5.07-6.28, comparing high or very high to very low overall morbidity), and the primary care provider's use of secure messaging with other patients (OR, 1.94; 95% CI, 1.67-2.26, comparing 20-50% vs. <or=10% encounters through secure messaging). Less secure messaging use was associated with enrollee age over 65 years (OR, 0.65; CI, 0.59-0.71) and Medicaid insurance vs. commercial insurance (OR, 0.81; 95% CI, 0.68-0.96). CONCLUSIONS: In this integrated group practice, use of patient-provider secure messaging varied according to individual patient clinical and sociodemographic characteristics. Future studies should clarify variation in the use of electronic patient-provider messaging and its impact on the quality and cost of care received.