Three patients presenting with both myasthenia gravis and thyrotoxicosis were treated initially with pyridostigmine and carbimazole respectively. Control of the hyperthyroidism was achieved in all cases but was accompanied by deterioration of the myasthenic symptoms in two and persistence in one. Thymectomy was performed with subsequent improvement in all three patients. Histology showed thymic hyperplasia in each case. The relationship of myasthenia gravis, thyrotoxicosis and the thymus is discussed.