Dear Ann Landers: Is it all right for a father to have sex with his daughter if both are consenting adults? I am 37 and my daughter, ''Jean,'' is 20. My wife passed away two years ago. After she became ill, Jean assumed all the household responsibilities and we became very close. We live here, just the two of us, and she is a wonderful cook and housekeeper. We have the same likes and dislikes and are compatible in every way.

Jean is pretty and I am still a young man. Rather than move out of a home that she loves and look for a husband (and risk getting AIDS), Jean says she wants to stay with me. We have a wonderful time together. She hasn`t dated in several months because she would prefer to be with me more than any of the young men she knows. I love her company and am not interested in anyone else. Can a man legally marry his daughter? Please do not publish my name as this is a very controversial subject and I don`t want to be harassed. P.S. Does the Old Testament say anything about this?

Mr. X.

Dear Mr. X.: According to John Beckstrom, professor of family law, Northwestern University Law School, it is not legal anywhere in the United States for a father and daughter to knowingly marry each other. Such a marriage would not be valid. In fact, in many states the father would be prosecuted for criminal incest.

You asked about the Old Testament: The Book of Leviticus makes it abundantly clear that it is a sin to approach ''any that is near of kin.''

Although you didn`t ask for my opinion, Mr. X., I feel compelled to say both you and your daughter sound as if you are two sandwiches short of a picnic. Your letter is one of the sickest I`ve read in years.

If you want Jean to have a decent life, please get her into counseling and find a woman who is suitable for marriage.

Dear Ann Landers: Here is my response to the Chicago Tribune Reader who asked you to print an open letter to her friends and relatives who are nagging her to quit smoking:

Dear Smoker: You say you know all about the health hazards of smoking and that you want your friends to stop bugging you to quit. You offered to make a deal. You promised to keep quiet when you see the third chunk of butter and a dollop of sour cream on your friend`s baked potato, or the scoop of ice cream on her piece of pecan pie. You feel it is a fair trade for not being harassed about smoking.

Sorry, dear, your reasoning is badly flawed. What I eat affects only me. Your secondhand smoke could cause cancer or heart trouble for those who have to breathe the air you pollute.

I am violently allergic to tobacco smoke and you have no right to inflict your vice on me. It`s perfectly all right with me if you choose to kill yourself, but I refuse to give up breathing while you`re doing it, so please take that stinking cigarette outside.

Healthy Lungs in Hayward, Calif.

Dear Hayward: Hundreds of readers had plenty to say about this, but you said it best and I thank you.

Gem of the Day: The high cost of health care these days is enough to make you sick.

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What can you give the person who has everything? Ann Landers` new booklet, ''Gems,'' is ideal for a nightstand or coffee table. ''Gems'' is a collection of Ann Landers` most requested poems and essays. Send a self-addressed, long, business-size envelope and a check or money order for $4.85 (this includes postage and handling) to: Gems, c/o Ann Landers, P.O. Box 11562, Chicago, Ill. 60611-0562. (In Canada, send $5.87.)