I would do:

lcDescr = thisform.ParentCombo.value
select (ComboRowSource)
set exact off
locate for Descript = m.lcDescr
if found()
  thisform.ChildCombo.ListIndex = recno()
endif

>>Hello All ,
>>
>>I have two combo boxes in a grid, all of which support incremental search.
>>One that has a list of category codes and the other detailed codes derived from an extention of the category codes.
>>
>>E.g main category code list (COMBO1)
>>GENTS-TOP
>>GENTS-BEL
>>
>>
>>Detailed code (COMBO2)
>>GENTS-TOP001
>>GENTS-TOP002
>>GENTS-TOP003
>>GENTS-BEL001
>>GENTS-BEL002
>>GENTS-BEL003
>>
>>
>>To help in the quick access to the detailed code i want the user to select a main category code in one combo
>>an then on clicking the detailed code combo i want to jump to position the first occurance of the main category (before locating the exact with a scrool or type)
>>Both combos use select SQL into a cursor.
>>
>>How can i implement such a thing.
>>
>>I'll appreciate some help.
>>
>>Thanks in advance.
>
>Sammy,
>Check out the 'Selected()' command.
>HTH,