In a letter that has stunned many leading fertility specialists, the acting head of their professional society's ethics committee says it is sometimes acceptable for couples to choose the sex of their children by selecting either male or female embryos and discarding the rest.

The group, the American Society for Reproductive Medicine, establishes positions on ethical issues, and most clinics say they abide by them.

One fertility specialist, Dr. Norbert Gleicher, whose group has nine centers and who had asked for the opinion, was quick to act it.

''We will offer it immediately,'' Dr. Gleicher said of the sex-selection method. ''Frankly, we have a list of patients who asked for it.''

Couples would have to undergo in vitro fertilization, and then their embryos would be examined in the first few days when they consisted of just eight cells.

Dr. Gleicher is chairman of the board of the Center for Human Reproduction, which has five fertility centers in the Chicago area and four others in Manhattan and Brooklyn. The acting chairman of the ethics committee at the reproductive medicine society, John Robertson, an ethicist and lawyer at the University of Texas, said he was responding to a request by Dr. Gleicher for clarification of the society's position. Mr. Robertson said that he wrote the letter after consulting with another committee member and that he thought it reflected the group's position.

The committee would have discussed the question at its meeting this month, he said, but the meeting was canceled because of the terrorist attacks. Mr. Robertson added that he expected the committee to discuss the letter at its next meeting, in January.

Mr. Robertson used the term ''gender variety'' to explain the acceptable uses of the sex selection technique. By that, he said, he meant that a couple who already had a child of one sex could ethically select embryos that would guarantee them that the embryo selected was of the opposite sex.

Mr. Robertson wrote that embryo sex selection could be offered for gender variety ''when there is a good reason to think that the couple is fully informed of the risks of the procedure and are counseled about having unrealistic expectations about the behavior of children of the preferred gender.''

The group's previous statement, in 1999, said that selecting embryos solely to have a child of a particular sex ''should be discouraged.''

Leading fertility specialists said they were taken aback by the new letter and could hardly believe its message.

''Sex selection is sex discrimination, and I don't think that is ethical,'' said Dr. James Grifo, the president-elect of the Society for Assisted Reproductive Technology, an affiliate of the reproductive medicine society. ''It's not ethical to take someone off the street and help them have a boy or a girl.''

Dr. Grifo, a reproductive endocrinologist at New York University Medical Center, added that publicity over centers offering such sex selection would sully the field and could ultimately make it impossible to help patients with a medical need for the technology.

''What's the next step?'' asked Dr. William Schoolcraft of the Colorado Center for Reproductive Medicine in Englewood. ''As we learn more about genetics, do we reject kids who do not have superior intelligence or who don't have the right color hair or eyes?''

The embryo selection method, called preimplantation genetic diagnosis, has been available for about a decade, but was reserved almost exclusively for couples at risk for having babies with certain genetic diseases. Doctors can test their embryos to see if they have the disease gene before implanting the embryos in the woman's uterus.

But it has always been clear that the method could easily be used for sex selection. It is simple to see if the embryo is male, with an X and a Y chromosome, or female, with two X chromosomes.

Fertility specialists say patients often ask if they can use preimplantation analysis solely to select the sex of their embryos. But most doctors say they refused.

''I could have financed my research from now until the day I die if I honored all the requests,'' Dr. Grifo said. ''But this is not the way we want this to go. We want to protect this technique for our genetic patients.''

Dr. Gleicher said he felt otherwise. He said he was prompted to ask about using preimplantation genetic diagnosis for sex selection when the reproductive medicine society issued a statement in May that it was ethical to use a different method -- sperm sorting -- for that purpose.

One center, the Genetics and IVF Institute in Fairfax, Va., is using a sperm-sorting technique that allows it to pick out the vast majority of male sperm. The reproductive medicine society said that if a couple already had one child and wanted to use the method to select the sex of subsequent children, that was ethically permissible.

But, Dr. Gleicher said, if sperm sorting is all right for sex selection, why prohibit preimplantation diagnosis for sex selection? ''How can you say that a method that would be 100 percent reliable is not ethically acceptable?'' he asked. He brought the question to his own ethics board.

''Our I.R.B. felt that it was unethical to offer an inferior method if a superior method is available,'' Dr. Gleicher said, referring to the institutional review board, an ethics committee.

But Dr. Schoolcraft said he saw a real difference between the two methods. ''With sperm sorting, you are not throwing away potential babies,'' he said.

Like Dr. Grifo, he worried about the future of his field. ''We have a responsibility to be conservative and cautious,'' Dr. Schoolcraft said. ''It's our responsibility not to misuse these technologies.''