I cannot see why the USE would not work. Do you get an error message when the program hits the line with USE? Also I would change the code to run like this;

SET PROCEDURE TO C:\myprog\program\nohabc.prg ADDITIVE   

use C:\myprog\data\helyseg.dbf exclusive

IF TAGNO( "NEV" ) > 0
  ALTER TABLE helyseg drop foreign KEY TAG NEV
ENDIF

RELEASE PROCEDURE C:\myprog\program\nohabc.prg

USE IN "helyseg"

>
>>>Hi all,
>>>I have a .prg file that calls another small file to get rid of the UDF on an index expression. It works fine, but if I want to close the altered table within this small file, it looks as if the "USE" command is skipped and ignored and the table stays open.
>>>If this small file is not called, run by itself, the command "USE" works.
>>>Why?
>>>
>>>Thanks
>>>
>>>Tamas Szigeti
>>
>>Hi Tamas. Can you post the code of the small file for everyone to review?