Influenza vaccine is a safe, effective, and cost-effective intervention that can prevent serious disease in adults. Opinions differ as to the most effective method for delivering the vaccine to the greatest number of high-risk adults. The objective of this article is to compare immunization delivery of influenza vaccine to high-risk adults during two types of clinic visits: routine scheduled appointments versus mass clinics. Data was collected at 15 ambulatory care settings on 599 patients 50 years and over from October 23, 2001, to January 31, 2002. Immunizations given at either routine scheduled visits or at mass influenza immunization clinics were compared for costs and resource requirements (productivity), and completeness of delivery of quality visit components (efficiency). The two visit types presented significantly different strengths on key clinical functions. Routine scheduled appointments promoted more review of patient health history and more of the contact information necessary for reminder/recall and audit functions. In mass immunization clinics, patients were more likely to be vaccinated, with far less time spent in either direct services or in waiting, and it was more likely that the required vaccination information statements (VIS) would be provided. Mass vaccination clinics and routine scheduled appointments are both viable service strategies for delivering influenza vaccines. This study suggests the greatest advantage occurs when both strategies are used in a coordinated manner.