OBJECTIVES: This study identifies factors that influence physicians’ use of medical resources. METHODS: We used the Healthcare Cost and Utilization Project (HCUP) State Inpatient Databases (SID), the American Hospital Association Annual Survey Database, and the Area Resource File in this analysis. Our hospital data for 2008 were drawn from Arizona and Florida, and physician information was obtained from medical boards of each state where we used physicians’ license numbers to register each hospital inpatient visit to a physician. Over 2.5 million inpatient records were used in the analysis. We employed linear cost models using all hospital inpatient stays registered to physicians for whom we had information on observable characteristics. We also estimated multilevel regression models that clustered hospital inpatient visits across physicians. We then repeated our linear regression analysis focusing separately on physicians working only in either teaching or non-teaching hospitals to address possible endogeneity of patient case mix and unobservable physician-specific factors that may not be completely addressed via regression models. Finally,  we re-estimated our multilevel model using all hospital inpatient stays registered to two subsamples of physicians based on their observable characteristics by employing propensity score nearest-neighbor (NN) matching without replacement. RESULTS: Our key findings remained the same across all estimations: 1) the costs of hospital inpatient stays registered to female physicians or foreign-trained physicians are significantly lower than the costs of hospital stays registered to male physicians or U.S.-trained physicians; 2) the costs of hospital stays registered to physicians with more experience is lower when compared to physicians with less experience; and 3) there is substantial variation in costs of hospital inpatient stays across board certified physician specialties, where surgeons and cardiologists are generally associated with higher costs of hospital inpatient stays. CONCLUSIONS: Our findings demonstrate that physicians’ characteristics have a significant impact on the costs of hospital inpatient stays.