The 48,XXYY syndrome is a form of sex chromosome aneuploidy presenting in 1:18,000 males. Tremor has been previously reported in 47,XXY and 47,XYY syndromes, but has not been well described in 48,XXYY syndrome. Ten males with 48,XXYY syndrome had a standardized neurological examination and videotaping, which included the Clinical Rating Scale for Tremor and the International Cooperative Ataxia Rating Scale. All 10 cases had postural and kinetic tremor on physical examination. Other findings included mild gait ataxia, dysarthria, and nystagmus. Three cases are reviewed. Tremor is a common finding in children and young adults with 48,XXYY syndrome. Dosage alteration of genes on the sex chromosomes may be involved in the pathogenesis of this tremor. Karyotyping should be considered in individuals presenting with tremor and a history of developmental delay, learning disabilities, tall stature, or microorchidism.