The aim of this study was to establish whether children with a physical disability resulting from central nervous system disorders (CNSd) show a level of arithmetic achievement lower than that of non-CNSd children and whether this is related to poor automaticity of number facts or reduced arithmetic instruction time. Twenty-two children with CNSd (M age = 10.7 years old) were compared with two groups of typically developing children: a same-aged group (n = 21) and a younger group (by 1 year) matched on arithmetic achievement (n = 23). Although their intelligence was in the average range, the arithmetic achievement level of the CNSd group lagged nearly 1.5 years behind their typically developing peers. There was no strong evidence that this was due to a specific automaticity deficit. However, the difference on an arithmetic achievement test between the CNSd group and same-aged control group was fully accounted for by the difference in hours of arithmetic instruction. © 2009 Hammill Institute on Disabilities.