Ten patients, three with postural tremor and seven with action myoclonus, had stereotyped involuntary rhythmic movements when attempting to execute a sustained isometric muscle contraction. The movements were characterized by rhythmic EMG bursts lasting less than 50 msec and appearing synchronously in agonist and antagonist muscles at a rate of 9 to 18 Hz. Backaveraging of the EEG activity related to the onset of the rhythmic EMG bursts identified a cortical potential preceding the EMG bursts in all patients. These symptoms and signs fit the description of “cortical tremor,” a variant of cortical reflex myoclonus. Cortical tremor is common in patients with cortical myoclonus and may be a source of functional disability. In two patients in whom we studied the effects of graded levels of isometric force, force recruitment modulated the abnormal EMG bursting frequency, amplitude, and spatial distribution of the myoclonic jerks in the activated limb. Transcranial magnetic and electrical stimulation, but not peripheral nerve stimulation, influenced the abnormal EMG bursting pattern, implying a greater dependence of this rhythmic phenomenon on a central generator than on peripheral feedback loops.