Williams syndrome (WS) is a rare genetic disorder accompanied by serious damage to numerical abilities. The working of two numerical systems, the analogue amount system and the verbal system, were investigated in Williams syndrome. Three different numerical tasks were used: simple addition, simple multiplication, and number comparison. A new matching method was introduced to find a proper control group. Typically developing control groups were matched using reaction time data from one of the tasks. The WS group had a relatively fast reaction time in addition and multiplication tasks (verbal recall of the number table), but the group was slow in number comparison (analogue quantity estimation in the intraparietal sulcus). In the number comparison task no reverse number effects were found, and the distance effect was stronger then in the control groups. On the basis of these data the postulation of an impaired analogue quantity system together with a relatively intact verbal recall system in Williams syndrome is supported.