Two patients with legs tremor present on standing, but none on walking or sitting, are reported. Tremor was not exclusive or orthostatism and was also evoked by strong tonic contraction of leg muscles. Synchronous EMG bursts were recorded in antagonistic muscle groups at 8-10 Hz in the first patient and at 16 Hz in the second. EMG activity was synchronous in corresponding muscles of both legs. The occurrence of EMG activity was not influenced by stimulation of nerve afferent fibers. We suggest that this movement disorder may be an exaggeration of physiological tremor due to synchronization of motor units by spontaneous oscillations in central structures.