As part of the "Provider and Patient Reminders in Ontario: Multi-strategy Prevention Tools" demonstration project, the purpose of this study was to explore the practice and physician characteristics associated with influenza vaccination rates following a reminder letter intervention for patients 65 years of age and older. Using a sample of 179 physicians, we estimated a multiple linear regression model to examine variables predictive of vaccination delivery rates. Several provider characteristics, including certification with the College of Family Physicians of Canada and practicing in an urban area, were predictive of the success of the reminder letter campaign. Examining other physician and practice factors associated with vaccination delivery following a reminder letter campaign may help improve such prevention efforts.