Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder occurring in male and rare female carriers of a premutation expansion (55 to 200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene.Volumetric MRI studies, clinical staging, cognitive testing, and molecular analysis were conducted in 15 female premutation carriers affected by FXTAS (age 59.5 +/- 10.3 years), 20 unaffected female carriers (43.3 +/- 11.2 years), 11 genetically normal female controls (51.0 +/- 10.3 years), 36 affected male carriers (65.0 +/- 5.6 years), 25 unaffected male carriers (53.5 +/- 12.5 years), and 39 male controls (58.0 +/- 15.0 years). Female and male carriers with FXTAS were matched on duration of disease.We found less pronounced reductions of cerebellar volume and a lower incidence of involvement (symmetric high T2 signal) of the middle cerebellar peduncles (MCP sign) in females affected by FXTAS (13%) compared with affected males (58%). We found reduced brain volumes and increased white matter disease associated with the presence of FXTAS in females compared with female controls. We also observed significant associations between reduced cerebellar volume and both increased severity of FXTAS symptoms and increased length of the CGG repeat expansion in male premutation carriers, but not in females.Females affected by fragile X-associated tremor/ataxia syndrome (FXTAS) demonstrated milder brain changes than affected males, although they showed a similar pattern of radiologic findings consistent with brain atrophy and white matter disease. FXTAS should be considered (by ordering fragile X DNA testing) in females who present with late-onset ataxia, action tremor, or neuropathy, particularly in those with a family history of mental retardation, autism, or premature ovarian failure.