Myasthenia gravis is the most common disease of neuromuscular transmission; however, it may be difficult to diagnose in the elderly patient with comorbid illnesses and vague symptoms. We report two elderly cases of MG, with a critical clinical course, in whom the diagnosis was missed by neurologists. A diagnosis of stroke was made in each of them, because of presence of abnormal cranial imaging findings. In one patient, the neurophysiological tests yielded negative findings at the initial examination. Clinicians should consider the diagnosis of MG, when faced with patients with new-onset weakness, especially of the cranial musculature and not be misled by the incidental neuroradiologic abnormalities. Confirmatory neurophysiologic tests are important, but should be repeated if the initial tests prove to be negative.