Fragile X-associated tremor/ataxia syndrome (FXTAS) develops in a subset of fragile X premutation carriers and involves gait ataxia, action tremor, Parkinsonism, peripheral neuropathy, autonomic disorders, and cognitive impairment. The study was designed to define the nature of cognitive deficits affecting male premutation carriers with and without FXTAS. A sample of 109 men underwent motor, cognitive, genetic, and neurologic testing, as well as brain magnetic resonance imaging. Subjects were classified into 3 groups: (a) asymptomatic premutation carriers, (b) premutation carriers with FXTAS, and (c) normal controls. Men with FXTAS performed worse than controls on mental status, intelligence, executive cognitive functioning (ECF), working memory, remote recall of information, declarative learning and memory, information processing speed, and temporal sequencing, as well as 1 measure of visuospatial functioning. Language and verbal comprehension were spared. Asymptomatic carriers performed worse than controls on ECF and declarative learning and memory. This comprehensive examination of cognitive impairment in male premutation carriers suggests that FXTAS involves substantial executive impairment and diffuse deficits in other cognitive functions. Longitudinal research currently underway will provide insight into the progression of the disorder.