Fragile X tremor/ataxia syndrome (FXTAS) is a recently described condition consisting of tremor, ataxia, parkinsonism, and executive dysfunction, presenting predominantly in male carriers of a fragile X mental retardation 1 premutation. In this report, we present premutation carrier sisters in whom severity of clinical signs correlated with a molecular pattern of X-inactivation favoring higher expression of the premutation allele. In these women with a common genetic background, we suggest that symptom severity may be dictated by X-inactivation, and thus a higher percentage of cells producing the premutation-containing mRNA result in increased toxicity and disease.