The clinical and physiological features of six new patients with primary orthostatic tremor are described. We suggest that use of the term primary orthostatic tremor be confined to the clinical syndrome in which unsteadiness when standing is the predominant complaint and accompanied by characteristic electrophysiological findings of a rapid (frequency around 16 Hz), regular leg tremor which is not influenced by peripheral feedback, is synchronous between homologous leg muscles, and in certain postures of the upper limbs, between muscles of the arm and leg. The fast frequency of muscle activity in primary orthostatic tremor of the legs causes unsteadiness when standing (presumably due to partially fused muscle contraction) but only a fine ripple of muscle activity is visible. In contrast, the slower frequency of other leg tremors, for example essential tremor, results in obvious leg movement which is evident in many leg postures, is variable over time and can be reset by a peripheral nerve stimulus. Essential tremor and orthostatic tremor do not respond to the same therapies, suggesting differences in the pharmacological profiles of the two conditions. Accordingly, there are clinical, physiological and pharmacological differences between primary orthostatic and essential tremor. Whether these factors are sufficient to regard these tremors as separate conditions is discussed.