BACKGROUND: A variable proportion of seronegative myasthenia gravis (SNMG) patients have antibodies to the muscle-specific tyrosine kinase (MuSK). Although several reports from Western countries suggest differences in the clinical features of MuSK antibody-positive and -negative SNMG patients, there have been no reports about these patients in Korea. METHODS: We performed the first survey of MuSK antibodies in Korea, measuring MuSK antibodies by commercial preparations (RSR Ltd) in the serum of SNMG patients who registered at the Seoul National University Hospital from October 2003 to January 2004, and identified clinical features and treatment responses prospectively until October 2004 using double blind method. RESULTS: Twenty-three (15 generalized and eight ocular MG, 15 men and eight women) SNMG patients with the ages from 1 to 60 years, (mean 36.24+/-16.82 years), were included. None of 8 ocular SNMG had MuSK antibody, whereas MuSK antibody was present in four (26.7%) of 15 generalized SNMG. All four MuSK positive patients were females, with pharyngeal and respiratory muscle weakness, and required immunosuppressive treatment in addition to acetylcholine esterase inhibitors. However, the overall disease severity and the age of onset did not show significant differences between MuSK antibody-positive and -negative SNMG patients and the responses to treatment were equally favorable. CONCLUSIONS: Our study showed the lower rate of MuSK antibodies in SNMG than the previous reports. However it seems to require large multicenter survey to confirm the possibilities of geographical or ethnical differences in the future.