Hereditary geniospasm is an unusual movement disorder causing episodes of involuntary tremor of the chin and the lower lip. Episodes typically start in early childhood and may be precipitated by stress, concentration, and emotion. Hereditary geniospasm is inherited as an autosomal dominant trait, and its cause is not known. We report the results of a genomewide genetic linkage study in a four-generation British family with hereditary geniospasm. Positive two-point LOD scores were obtained for 15 microsatellite markers on the peri-centromeric region of chromosome 9. A maximum two-point LOD score of 5.24 at theta = .00 was obtained for the marker D9S1837. Construction of haplotypes defined an interval of 2.1 cM between the flanking markers D9S1806 and D9S175, thus assigning one locus for hereditary geniospasm to the proximal long arm of chromosome 9q13-q21. Hereditary geniospasm in a second British family is not linked to this region, indicating genetic heterogeneity. These findings may have implications for other inherited focal movement disorders that as yet remain unmapped.