>How would you insure that the entries in a given field are unique OR blank? That is, multiple blank records are OK but if data is entered, it must be a unique value. Setting up a candidate index doesn't work due to the blank fields.

Don,

Can you use NULL instead blanks? Are you using buffering?

You can do something like

lnRecord = recno() && Save current record pointer
set order to myField
if seek(myNewValue)
  scan while myField = myNewValue
     if recno() <> m.lnRecord  && Duplicate
       exit
   endif
  endscan
endif

Also may be we can set a candidate index with the FILTER? Try FOR not empty(FIELD). Just an idea.