Background: Antibodies (Ab) against the acetylcholine receptor (AChR) are found in 80-90% of generalized myasthenia gravis (MG),but only in 50% of pure ocular MG. Furthermore,Ab against the muscle-specific receptor tyrosine kinase (MuSK) were detected in 38-54% of patients with AChR-Ab negative (“ seronegative” ) MG,but not in pure ocular MG. Methods: 2 case reports have been analysed: Two patients (42 years old and 61 years old) with primary ocular MG were studied. Both patients complained of fluctuating bilateral ptosis and double vision lasting for 2 to 3 months. Results: AChR-Ab were negative but MuSK-Ab were clearly positive. In spite of treatment with pyridistigmin and prednisolone,the symptoms in patient 1 generalized within 4 weeks causing dysphagia,limb muscle weakness and respiratory crisis. Symptoms immediately responded to plasmapheresis. Patient 2 clearly improved under treatment with pyridostigmine,methylprednisone,and azathioprine. There was no generalizationwithin a course of 12 months. Conclusions: Both cases clearly indicate that MuSKAb can be found in primary isolated ocular MG lasting for 2 to 3 months. In a primary ocular manifestation of MG the detection of MuSK might indicate the risk for subsequent generalization and might have implications for immunosuppressive therapy. Therefore,seronegative MG requires testing for MuSK-Ab. Background: Antibodies (Ab) against the acetylcholine receptor (AChR) are found in 80-90% of the generalized myasthenia gravis (MG), but only in 50% of pure ocular MG. Further, Ab against the muscle-specific receptor tyrosine kinase MuSK) were detected in 38-54% of patients with AChR-Ab negative (“seronegative”) MG, but not in pure ocular MG. Methods: 2 case reports have been analyzed: Two patients (42 years old and 61 years old) Both patients complained of fluctuating bilateral ptosis and double vision lasting for 2 to 3 months. Results: AChR-Ab were negative but MuSK-Ab was clearly positive. In spite of treatment with pyridistigmin and prednisolone, the symptoms in patient 1 generalized within 4 weeks causing dysphagia, limb muscle weakness and respiratory crisis. Symptoms immediately responded to plasmapheresis. Patient 2 clearly improved under treatment with pyridostigmine, methylprednisone, and azathioprine. There was no generalization with in a course of 12 months. Conclusions: Both cases identify indicate that MuSKAb can be found in primary isolated ocular MG lasting for 2 to 3 months. In a primary ocular manifestation of MG the detection of MuSK might indicate the risk for subsequent generalization and might have implications For immunosuppressive therapy. Thus, seronegative MG requires testing for MuSK-Ab.