Purpose: This study was intended to investigate the frequency of job performance of the Korean professional medical support staffs (PMSS). Method : The data of 1,666 PMSS from 36 hospitals and over 500 beds were analyzed. The participants were divided into 5 groups: advanced practice nurses (APN), clinical nurse experts, physician assistants (PA), coordinators, and others. Results : Among the 5 main domains of job performance, advanced clinical practice has the highest frequency (111.36 d/y), followed by consultation/collaboration (75.66 d/y), education/counseling (53.54 d/y), leadership (23.90 d/y), and research (19.14 d/y). There was a significant difference in the frequency of job performance between the 5 groups of participants. The invasive activities were more frequent in the PA group. In the education and counseling domain, APNs had a higher level of job frequency than others (p<.001). In the research and leadership domains, APNs and coordinators had more prominent performance frequency than other groups (p<.01). However, there are some ambiguities in the job performance of the 5 groups depending on institutional characteristics. Conclusion : To establish the scope of work of PMSS, organizational and individual efforts are needed to promote and expand the leadership and research domains. To resolve the ambiguities of PMSS' roles, it is necessary to reorganize their titles. [ABSTRACT FROM AUTHOR]