Current neurocognitive theories of number processing [Dehaene, S., Piazza, M., Pinel, P., & Cohen, L. (2003). Three parietal circuits for number processing. Cognitive Neuropsychology, 20, 487-506] state that mathematical performance is made possible by two functionally and anatomically distinct subsystems of number processing: a verbal system located in the angular gyrus, which underlies the retrieval of arithmetic facts, and a quantity system located in the intraparietal sulcus, which subserves operations that involve semantic manipulations of quantity. According to this model, subtypes of math disability (MD) should be traceable to differential impairments in these subsystems. The present study investigated MD in children with velo-cardio-facial syndrome (VCFS) and aimed to verify which of these subsystems of number processing is impaired in these children. Eleven children with VCFS and 11 individually matched controls, selected from the same classes, completed a large battery of mathematical tests. Our data revealed that children with VCFS had preserved number reading abilities and preserved retrieval of arithmetic facts, both of which indicate that the verbal subsystem is not impaired in VCFS. By contrast, children with VCFS showed difficulties in number comparison, the execution of a calculation strategy and word problem solving, all of which involve the semantic manipulation of quantities. This provides evidence for a specific deficit in the quantity subsystem in children with VCFS, suggesting underlying abnormalities in the intraparietal sulcus.