How health care providers compete and how competition among them affects their behavior are crucial questions in theory and health policy. In ordinary markets, competition improves social welfare. However in health care markets facing uncertainty and information asymmetry, competition can take the form of wasteful quality competition and result in cost increase. The purpose of this study is to examine the characteristics of hospital service markets and examine the impact of hospital competition on hospital behavior, more specifically hospital cost and the size of personnel. Based on patient discharge data of 2002 by the Ministry of Health and Welfare and Korea Institute for Health and Social Affairs, and health insurance EDI claims data of 2002, this study measures the degree of competition in the inpatient service market of hospitals, using variable radius method and Herfindahl index. The result of the study shows that the hospital service market consists of on average 3.13 government administrative units(shi, gun, or gu). Compared with hospitals, general or general specialized hospitals cover larger markets and operate in more competitive markets. Nearly 60% of patients use hospitals, which are not located in their government administrative units, meaning that market definition based on variable radius is better than the conventional method of market definition based on government administrative units. The results of multivariate analysis show that competition is not associated with high cost index of hospitals. But hospitals in more competitive markets employ larger(more intensive) input of personnel per 100 beds, implying that hospital competition in Korea can have the form of quality and cost-increasing competition.