Description: During last decade sporadic combination of multiple sclerosis (MS) and myasthenia gravis (MG) has been reported repeatedly. Although they seem anecdotal, they are importance enough to rise concerning about co-occurrence of MG and MS. Here we presented two cases of MS who developed MG during their disease period. Both of them have been received interferon with diagnosis of relapsing remitting MS. Interestingly, one of them developed MG crisis 4 years after diagnosis of MS. MS and MG have relatively same distribution for age, the younger peak of the bimodal age distribution in MG and HLA typing. Furthermore, some evidences support the role of systemic immune dysregulation due to a genetic susceptibility that are common to this group of diseases. The association may be underdiagnosed because of the possible overlap of symptoms specially cranial manifestations in which either MG or MS can mimic each other manifestations leading to underestimating incidence of new ones. The evident warrant physician specially neurologist to always consider presumably occurrence of another disease when encounter with such a patients either with MS or MG