Background.  The ability to compare numbers, as the most basic form of number sense, has been related to arithmetical achievement.

Aims.  The current study addressed the predictive value of non-symbolic and symbolic (number word (NW) and Arabic number (AN)) comparison for arithmetics by means of a longitudinal design.

Sample.  Sixteen children with mathematical disabilities (MD), 64 low achievers (LA), and 315 typical achieving (TA) children were followed from kindergarten till grade 2.

Method.  The association of comparison skills with arithmetical skills in grades l and 2 was studied. The performances of MD, LA and TA children were compared.

Results.  Regression analyses showed that non-symbolic skills in kindergarten were predictively related to arithmetical achievement 1 year later and fact retrieval 2 years later. AN comparison was predictively related to procedural calculation 2 years later. In grade 2, there was an association between both symbolic tasks and arithmetical achievement. Children with MD already had deficits in non-symbolic and symbolic AN comparison in kindergarten, whereas in grade 2 the deficits in processing symbolic information remained.

Conclusions.  The combination of non-symbolic and symbolic deficits represents a risk of developing MD.