Sustained attention and response inhibition were examined in boys with full mutation fragile X syndrome (FXS) using adapted visual and auditory continuous performance tests (CPTs). Only 61% of 56 boys with visual CPT data and 54% of 52 boys with auditory data were able to demonstrate sufficient understanding to complete the visual and auditory CPTs, respectively. Mental age (MA) predicted whether boys with FXS were able to demonstrate understanding of the CPTs. The performance of boys with FXS who were able to complete the CPTs was compared to a sample of boys without disabilities matched on MA. Boys with FXS demonstrated similar or smaller declines in sustained attention over task time than their MA-matched peers on the visual and auditory CPTs, respectively, but consistently demonstrated greater declines in response inhibition over task time than their MA-matched peers. There were no differences between groups for response time of hits. Higher MAs consistently predicted better sustained attention and response inhibition over task time on the visual and auditory CPTs. Furthermore, boys taking psychotropic medication performed better at the beginning of most tasks, although their performance deteriorated at a faster rate over time, and boys rated as meeting diagnostic criteria for ADHD-hyperactive type had more difficulty over task time with response inhibition on the auditory CPT. For both boys with FXS and their MA matches, performance was better on the visual CPT than on the auditory CPT though this effect may be attributable to a number of factors other than the modality. © 2007 Wiley-Liss, Inc.