Six patients in two unrelated families had a hereditary form of sustained muscle activity of peripheral nerve origin. Although varying in severity among affected family members, the disease was manifested clinically as generalized myokymia and muscle stiffness. Motor and sensory nerve conduction velocities and results of nerve biopsy were normal. Repetitive after-discharges followed each motor nerve stimulus. Needle electrode examination demonstrated spontaneous recurrent bursts of motor unit activity. The burst activity of a motor unit occurred rhythmically (0.5 to 4 bursts per second) and independently of the activity of other units. Within a burst, the firing rate was high (150 to 300 Hz). The clinical and electromyographic abnormalities improved considerably after treatment with carbamazepine or phenytoin.