A distinct syndrome with continuous motor neuron discharges apparently developed in seven members of a single family, involving both sexes and spanning three generations. Persistent vermiform twitching and episodic stiffness predominantly in lower extremity muscles occurred in early childhood and tended to be less severe in adulthood. In 2 patients the clinical manifestations improved with oral phenytoin and carbamazepine but not with parenteral diazepam. Insertional activity was normal, and continuous, rhythmical, normal-appearing muscle discharges were observed on electromyography. The cerebrospinal fluid levels of homovanillic acid and 5-hydroxyindoleacetic acid were increased in the proband. The disappearance of continuous muscle discharges during spinal anesthesia and the lack of response to diazepam indicated generation of the discharges from the proximal portion of the motor unit.