Williams' syndrome (WS) is a rare, genetically based disorder of cognitive development. Affected individuals show a severe deficit of spatial cognition but a relative sparing of language and face recognition. To examine the possible neural basis of the spatial deficit, we tested a group of WS children, aged 4-14 years, on two measures specific to dorsal cortical stream function: global motion coherence thresholds, in comparison with an analogous form-coherence test, and visuo-manual accuracy in posting a card through a slot, compared with matching the slot orientation. Deficits in these tasks provide the first evidence of specific involvement in WS of the dorsal stream, the cortical system believed to encode information about spatial relationships and the visual control of action.