Purpose: The unfortunate association between myasthenia gravis and Graves' disease is not widely appreciated. It would be consistent with a genetic predisposition for autoimmune disease. The frequency of this association is variously appreciated in the literature. Case: We report a case of a 20 years old patient with a two years history of Graves' disease, who presented with a myasthenic syndrome leading to an acute respiratory failure. This was found to be reversible with prostigmine, and the diagnosis of myasthenia gravis (MG) was confirmed by EMG and positive immunology. There was a positive response to medical treatment. Discussion: The frequency of this association is variously appreciated in the literature. MG has been reported to be discovered simultaneously with, or prior to, the diagnosis of Graves' disease, but is most commonly subsequent to it. The similarity of symptoms makes diagnosis difficult. Furthermore, an excess of thyroid hormones worsens MG and the existence of MG imposes certain precautions in the management of hyperthyroidism. Conclusion: Research is called for into the surveillance and management of specific manifestations of the co-occurrence of these conditions.