Symptomatic and essential rhythmic palatal myoclonus. Rhythmic palatal myoclonus (RPM) is a rare movement disorder consisting of continuous synchronous jerks of the soft palate, muscles innervated by other cranial nerves and, rarely, trunk and limb muscles. It usually develops secondary to brainstem or cerebellar disease (symptomatic RPM). Some patients, however, fail to show evidence of a structural lesion (essential RPM). A total of 287 cases with RPM from the literature including 210 cases with symptomatic and 77 cases with essential RPM have been reviewed and analysed statistically to look for criteria separating the two conditions. Patients with essential RPM usually have objective earclicks as their typical complaint which is rare in the symptomatic form. Eye and extremity muscles are never involved. The jerk frequency is lower in essential than in symptomatic RPM. Patients with essential RPM are younger and have a balanced sex distribution as compared with a male preponderance in the symptomatic form. The rhythmicity of RPM seems to be more profoundly influenced by sleep, coma and general anaesthesia in essential than in symptomatic RPM. We conclude from these results that essential RPM should be separated as a distinct clinical entity. Symptomatic RPM is a rhythmic movement disorder whose pathogenesis is quite well established. The cells of the hypertrophied inferior olives are believed to represent the oscillator. Among other possibilities, essential RPM may represent its functional analogue, based on transmitter changes only. Such a relationship could be of theoretical interest for the understanding of rhythmic hyperkinesias in general. 