Graves disease (GD) and myasthenia gravis (MG) may be associated and influence one another clinical expression. We report the cases of 3 female patients who presented this particular association. First patient (RL, 55 years), was diagnosed with MG and, 5 years after, developed GD, remitted after 2 years of anti-thyroid drugs (ATD). She is still euthyroid after more than 10 years, and MG is compensated with moderate doses of anticholinesterase drugs (ACD). Thoracic CT showed stable thymus hyperplasia. Second patient (AI, 42 years) have been diagnosed with both diseases practically in the same time: suspicion of MG, positive electromyography, clinical symptoms suggesting GD with biological confirmation. She had normal thoracic CT. She had a more difficult evolution, with cardiac failure, rapid installation of hypothyroidism on medium doses of ATD and persistence of important neuro-muscular symptoms, which made necessary near-total thyroidectomy. Post-surgical she presented higher LT4 necessary (200 mcg/d) but MG was spectacularly improved. Third patient (MM, 56 years) had a first episode of GD and, 5 years later, relapse of GD and onset of MG. Near-total thyroidectomy was performed, with post-surgical hypothyroidism compensate with 75 mcg LT4. However, MG symptoms persisted, and needed higher doses of ACD. Thoracic CT showed thymus tumour which was resected. The histology showed thymolipoma and the necessary ACD doses remained unchanged. The association of GD and MG is challenging because the evolution of one disease may be influenced by the other. Two of our patients had an improvement of the MG evolution after remission of GD but neither thyroidectomy nor thymectomy could diminish the ACD needs for the third. The diagnosis of MG had preceded, followed or been contemporary to the GD diagnosis. No matter who comes first, one should have in mind this association and adjust the management to the specific evolution.