Attentional problems are commonly reported as a feature of the behavioural profile in both Williams syndrome (WS) and Down's syndrome (DS). Recent studies have begun to investigate these impairments empirically, acknowledging the need for an approach that considers cross-syndrome comparisons and developmental changes across the different component functions of attention. The present study assessed children with WS and DS using a new preschool attention battery (ECAB: early childhood attention battery), designed to be suitable for mental age 3–6 years including groups with developmental disorders. The ECAB has the advantage of giving an individual profile of attentional abilities for each child, covering different components of attention. In relation to test norms for their mental age, both groups showed a profile of strengths and weaknesses in the attention domain. Both syndrome groups performed relatively well on tests of sustained attention and poorly on aspects of selective attention and attentional control (executive function). The DS group showed a specific strength in auditory sustained attention, whilst the WS group showed a particular deficit in visuo-spatial response control. There was also evidence for considerable differences in the developmental trajectory of these abilities across the two groups. The results provide evidence for syndrome-specific patterns of impairment, and distinct profiles of strengths and weaknesses that may be useful in understanding the nature of everyday attention difficulties in these groups and tailoring interventions to meet these needs.