To the Editor: In a letter in the October 9 issue, Dr. Stanley Fahn wrote that the levodopa provocative test for Huntington's disease (HD) should not be used. I agree with his conclusion on the bases of validity and safety, but I strongly disagree with his opinion that the results of any presymptomatic test for HD should be withheld from all the subjects involved.The assumption implicit in Dr. Fahn's reasoning is that it is better for persons to live with the knowledge that they have a 50 per cent chance of having HD than to live with the certain . . .