Influenza is a serious disease, especially for older people, and incomplete vaccination take-up poses a major public health challenge. On both the side of physicians and patients, there could be promising channels for increasing immunization rates, but no attempt has yet been made to empirically unravel their respective influences. Using exclusion restrictions implied by an economic model of physician-patient interactions, our study quantifies the particular effects of supply and demand on influenza immunization. On the supply side, our estimates highlight the importance of physician agency and physician quality, while a patient's education and health behaviors are key demand side factors.Keywords: Influenza Vaccinations, Supply and Demand, Semiparametric Methods * Mannheim Research Institute for the Economics of Aging (MEA), University of Mannheim, L13, 17, 68131 Mannheim, Germany and Health, Econometrics and Data Group (HEDG), University of York, United Kingdom. Email: maurer@mea.uni-mannheim.de. I am grateful to seminar and conference participants at the Mannheim Research Institute for the Economics of Aging, the Centre for Health Economics at the University of York, the SHARE user conference in Chania, the National University of La Plata and the RAND Corporation in Santa Monica for helpful comments and suggestions. Moreover, I should like to thank Frank Vella for his input regarding the estimation of the empirical model and Vincent Pohl for sharing his data with me. Of course, all remaining errors are my own responsibility.