The occurrence of thyrotoxicosis in myasthenia gravis is estimated as 5.3% (Osserman et al. 1967) but the incidence of myasthenia in the course of thyrotoxicosis is only 0.2% (Simpson 1968). The association between these two diseases is not a chance one and there are two possible basic mechanisms viz. the action of thyroid hormone on the neuromuscular junction or the existence of a cause common to both. The fatigability seen clinically in thyrotoxic myopathy could be partly attributed to myasthenia. With these possibilities in mind, a search for electromyographic stigmata of latent myasthenia was done on 48 consecutive cases of hyperthyroidism who also had chronic myopathy by performing repeated nerve stimulation. Eight cases had abnormal neuromuscular transmission as in myasthenia gravis and one of these cases later developed clinical myasthenia involving the ocular muscles. Three other cases showed abnormal facilitation as in the Eaton-Lambert syndrome.