Physicians' treatment decisions determine the level of health care spending to a large extent. The analysis of physician agency describes how doctors trade off their own and their patients' benefits, with a third party (such as the collective of insured individuals or the taxpayers) bearing the costs. Professional norms are viewed as restraining physicians' self-interest and as introducing altruism towards the patient. We present a controlled experiment that analyzes the impact of professional norms on prospective physicians' trade-offs between her own profits, the patients' benefits, and the payers' expenses for medical care. We find that professional norms derived from the Hippocratic tradition shift weight to the patient in the physician's decisions while decreasing his self-interest and efficiency concerns.Keywords: social preferences, allocation of medical resources, professional norms JEL classification: A13, I19, C72, C91 * We would like to thank seminar audiences at Aarhus University, the Harvard Health Care Policy Center, the Kelly School of Business, and the Universities of Copenhagen, Michigan, and Stirling, as well as participants of the 2014 Workshop on Behavioral and Experimental Health Economics at the University of Oslo for valuable comments and discussions. We would also like to thank Suzan Elshout and the team of programmers at CentERdata Tilburg for implementing the internet experiment; René Cyranek and the whole MELESSA team for supporting the fieldwork; and Hendrik Brackmann for excellent research assistance. We are grateful to Martin Fischer and Matthias Holzer for their help in fielding this study with a sample of Munich medical students. We acknowledge financial support by the Deutsche Forschungsgemeinschaft through SFB/TR 15.