We report the data on effects on visual search of (1) preview displays and (2) singleton targets and distractors in IQ-matched ADHD and control children. All children showed interference from singleton distractors even when targets never carried singleton values. This interference from singleton distractors increased under preview conditions, indicating that the children then had fewer resources available to control attention. There was also one selective deficit for ADHD children; they showed marked problems in responding to singleton targets following preview displays. This suggests that, in ADHD, there is either a selective delay or an impairment in switching attentional sets (from a negative set to the preview to a positive set to a singleton target). We discuss the implications for understanding both ADHD and the development of selective attention.