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SUMMARY Twenty-eight cases of mirror writing were seen during a period of three and a half years. These consisted of 12 patients with essential tremor, nine with Parkinson's disease, three with spino-cerebellar degeneration and four other cases. There were no cases of hemiparesis, aphasia, apraxia, agnosia or confusion. Fragmentary reversals were excluded from this study. Since essential tremor, Parkinsonian tremor and cerebellar tremor can be abolished by a stereotaxic produce applied to the thalamus, a common neural pathway via the thalamic nuclei may exist in these disorders. The existence is therefore proposed of some neural mechanism that controls the higher cerebral function of writing via the thalamus. were unskilfully written and drawn because of his action tremor. When he was instructed to write with the left hand, however, he wrote kanji from right to left in mirror fashion without any hesitation. He did not even recognise his own mirror writing. In the case of the clock face, not only the numerals, but also the position of the clock digits were reversed. Prospective Evaluation of the Clinical Cases Following our discovery of this initial case, we decided to evaluate the incidence of mirror writing in cases of neurological disorders which included not only essential tremor, but also Parkinson's disease, spinocerebellar degeneration, cerebrovascular disease, and various other neurological complaints, on the basis of work in an outpatients clinic. In addition, 258 medical and nursing students were asked to cooperate by writing their names with both right and left hands without any further suggestions. Although these students were much younger than the patients and were therefore unsuitable as perfectly exact age and sex matched controls, they can nevertheless be used as examples of how normal subjects write with their left hands.The mirror writing we report on here was, of course, induced mirror writing, which is somewhat different from the many types of case with spontaneous mirror writing classified by Critchley. Yet in every case, mirror writing was demonstrated when our patients were instructed to write letters with their left hands.