In this prospective study, 27 patients with hyperthyroidism were evaluated neurophysiologically. There were 24 women and 3 men: the mean age was 56.4 years (range 21 to 84 years). Neurologic complaints of the patients were muscle weakness (59.3%), muscle cramps (18.5%) and sensory symptoms (11.1%). Tendon reflexes were increased in 18.5% and decreased in 14.8%, respectively. While the motor nerve conduction velocities (NCV) of the tibial and the peroneal nerve revealed normal results in the majority of patients, there were pathological findings of the sensory NCV of the sural nerve in 29.6% of patients. In another 22.2%, borderline findings were obtained. Six patients presented myopathic changings in electromyography of the m.vastus lateralis. Neither neuropathic nor myopathic findings correlated with the serum levels of TT3, TT4 or the T4/TBG-quotient. A polyneuropathy in hyperthyroidism seems not to be uncommon and is probably caused by hypermetabolism in thyroid hyperfunction.