The main objective of this paper is to investigate the factors affecting organizational citizenship behavior of hospital employees, and based on the investigation, to suggest some implications for effective human resources management of hospitals. For this purpose, physical therapists were selected as the subject of the research. Using their individual characteristics, job characteristics, professionalism and group cohesiveness as the variables affecting organizational citizenship behavior, an empirical model was constructed and tested. A survey was conducted through structured and self-administered questionnaire for the physical therapists working at hospitals of Busan-Kyongnam area, and data from 240 therapists were utilized in the final analysis. Major results of the empirical analysis are as follows: First, perception on professionalism and the degree of organizational citizenship behavior were higher for male, older, relatively more-educated and higher-grade employees. It is necessary to develope some measures to educate and motivate the employees who are in lower state of professionalism and organizational citizenship behavior. Second, among the individual characteristics, need for growth was found to have significant, positive influence on professionalism and group cohesiveness, but no direct effect on organizational citizenship behavior. On the other hand, extroversion had direct, positive effect on organizational citizenship behavior, as well as on professionalism and group cohesiveness. This result suggests that personnel selection and personality education should be conducted carefully. Third, job characteristics appeared to have very large, positive effect on professionalism, but not directly on organizational citizenship behavior. Fourth, professionalism was found to have very large, positive influence on group cohesiveness and direct, positive effect on organizational citizenship behavior. This implies that enhancing professionalism of physical therapists can strengthen organizational citizenship behavior in hospitals, and hence top management should actively support the programs for job re-design, skill education and quality improvement to enhance professionalism of their employees. Fifth and last, as an intervening factor, group cohesiveness appeared to have the largest, direct, positive effect on organizational citizenship behavior. It is, therefore, important for top management to improve group cohesiveness by exploring ways toward greater harmony and solidarity among the members of physical therapy department.