The co-occurrence of myasthenia gravis (MG) and Guillain Barré syndrome (GBS) is uncommon with a few reported cases in the literature. There is only one reported case of MG and Miller Fisher variant of GBS. We described an 84 year old Chinese woman with underlying seropositive myasthenia gravis (MG) who presented with ophthalmoplegia, areflexia and acute neuromuscular weakness. She was proved to have co-occurrence of MG and GBS/Miller Fisher overlap syndrome with positive anti-GQ1b antibody. The unusual finding in this patient raises an interesting question on their pathogenesis with the possibility that prior activation of the immune system may predispose the development of autoantibodies against other antigens within the same set of muscles.