Previous research of the behavioral phenotype in fragile X syndrome (FXS) has found that boys with FXS often have problems with inattention, hyperactivity, and impulsivity; furthermore, recent research has found many boys with FXS have symptoms of attentiondeficit/hyperactivity disorder (AD/HD). Similarly, neuropsychological studies in boys with FXS have found deficits in sustained attention and inhibitory control. However, due to minimal research in this area, lack of appropriate comparison groups, and inconsistent measures, these results have yet to clarify the nature and severity of these deficits in boys with FXS. Similarities found in these disorders (i.e., FXS, AD/HD) allow an examination of these two groups to delineate AD/HD symptoms in males with FXS. This study examined sustained attention and inhibition in 57 boys with full mutation FXS, 48 boys without disabilities matched on mental-age (MA) to the boys with FXS, and 30 boys diagnosed with AD/HD using adapted visual and auditory continuous performance tests (CPTs), a standardized measure of sustained attention from the Leiter-R, and an experimental Day/Night task of inhibition. The performance of the boys with FXS who demonstrated sufficient understanding of the CPTs in order to complete the tasks was compared to the boys without disabilities, matched on mental age, and boys verified to have a diagnosis of AD/HD. Boys with FXS consistently demonstrated greater declines in inhibitory control and sustained attention over the length on the auditory and visual CPTs than the other groups. Regarding response time to hits on the CPTs, boys with FXS responded significantly slower to targets on the visual CPT, while groups did not differ on the auditory CPT. Similarly to the CPTs, the boys with FXS demonstrated greater difficulty inhibiting their automatic response on the experimental Day/Night task than the boys with AD/HD and the MA-matched peers. On the standardized measure of sustained attention, there were no significant differences between the boys with FXS and the boys with AD/HD. Furthermore, no child or family variables predicted performance on the measures of sustained attention and inhibition in boys with FXS.