Twenty index patients with hereditary essential tremor and their kindreds were studied to define the phenotype of this condition. Ninety-three first degree and 38 more distant relatives were examined; 53 definite and 18 possible secondary cases were identified. The age of tremor onset was bimodally distributed with a median at ˜15 years. Segregation analysis indicated autosomal dominant inheritance and penetrance was virtually complete by the age of 65 years. There were no examples of the disease skipping a generation. Men and women were affected in equal proportions. About 50% of cases were alcohol responsive. In the majority of families alcohol responsiveness was either consistently present or did not occur, but in 20% of kindreds definite heterogeneity of responsiveness was encountered within each family. The typical phenotype was a mild symmetrical postural tremor of the upper limbs. Tremor of the legs, head, facial muscles, voice, jaw and tongue occurred but never in isolation and rest, task specific (e..rimary writing tremor) and primary orthostatic tremors were not found. Head tremor was invariably mild and 75% was of a ’no-no‘ type. Dystonia (e.g. torticollis and writer's cramp) were not encountered, a finding which strongly suggests that many previous studies of ‘essential tremor’ were contaminated by cases of idio-pathic or hereditary torsion dystonia. No association with Parkinson's disease was found but classical migraine occurred in ˜26% of cases and co-segregated with tremor. The severity of arm tremor (assessed using a clinical rating scale and by scoring tremor in Archimedes spirals) and disability increased with advancing age and increasing tremor duration, but there was no correlation between age at tremor onset and either tremor severity or disability. Men and women were affected with equal severity. The sex of the affected parent had no influence on the severity of tremor or the degree of disability experienced by an affected child. Disability commenced in the second decade and progressively increased. All the index patients and 59% of the definite secondary cases had tremor induced disabilities. Eighty-five percent of index patients and 38% of secondary cases also reported some degree of social handicap. Twenty-five percent of index patients and 12% of secondary cases had been compelled to change jobs or retire. Biological fitness was normal.