This paper presents a detailed analysis of one aspect of performance by young males with fragile-X syndrome (FMR-1 full mutation) who were assessed on a computerised visual search task as part of a larger study examining aspects of attention [Neuropsychologia 38 (2000) 1261]. They were matched on chronological and mental age to 25 boys with Down's syndrome (trisomy 21) and on mental age to 50 mainstream school boys (controls). The controls were further divided into those matched on "poor attention" to the fragile-X boys and a "good" attention group, as rated by the comprehensive teacher rating scale (ACTeRS) questionnaire. Both fragile-X and Down's syndrome boys made significantly more repeated responses on targets (but a lower proportion of errors based on confusion of shape) than the two control groups and these differences were stronger in the fragile-X group. In the single target condition, search was for a single type of target throughout. In the dual target condition, participants were required to alternate between two different targets. Fragile-X boys showed significantly greater inability than Down's syndrome and normal boys to switch attention between targets and both learning-disabled groups were inferior to the control groups. Thus, both learning-disabled groups displayed a weakness in inhibiting repetition and in switching attention from one type of target to another and the impairments were more acute in fragile-X boys. The results provide further support for an attention deficit in this population at higher levels of attention control/executive functioning that involve switching visual attention and inhibiting repetitious behaviour.