THEY ENTER THE SOULLESS brown mid-rise in silence, strangers in a Fairfax office park, eager to remain anonymous, ready to play God. Smiling professionals usher them quietly past rows of cubicles -- this could be a mortgage company, or a telemarketer -- and into a seminar room.

Forty people in their thirties and forties. Would-be parents who want to subvert a basic rule that has stood throughout the history of living things: When it comes to boy versus girl, you take what comes. Not anymore. This is the headquarters of the Genetics-IVF Institute, where the barriers of reproductive technology are routinely smashed. The institute's latest product is MicroSort, a machine that sifts through sperm and separates the X's from the Y's, allowing customers to choose whether to have a boy or a girl.

The process isn't quite foolproof -- in trials thus far, 93 percent of couples who wanted a girl have gotten their wish and 73 percent of couples seeking a boy got one -- but it's convincingly better than God's even odds. Price: $2,500 per menstrual cycle (on average, it takes four cycles to achieve pregnancy).

Nature's way isn't faring too well in reproduction these days; science is gaining in many ways. In the past decade, the proportion of women who pick their delivery date by having their doctors induce labor has doubled to 18 percent. Geneticists can screen fetuses for dozens of diseases, allowing parents to decide whether to let nature take its course or halt the process and try again. The infertility industry has become a robust profit center in medicine, extending women's childbearing years almost unimaginably.

But gender selection is a big step. It's the closest we've come so far to designer kids.

Edward Fugger, the biologist who developed MicroSort, based on U.S. Department of Agriculture research on animals, shows nifty slides that look like constellations at an extremely hip planetarium -- foggy splotches of light, green for Y chromosomes and pink for X chromosomes. Fugger's sorting machine is based on a simple fact: X chromosomes have more DNA. Measure the DNA in each sperm cell, and voila -- sex selection.

Throughout this introductory evening, the questions focus not on the science, but on the ethics and security of the process. Several people fixate on the danger of sample-swapping. One man even wants to watch his ejaculate as it moves through the lab.

But the evening really gets interesting when a woman asks, "What about a couple that has no children and wants one of a particular sex?" Fugger stiffens. "At this time, that's not a criterion for this trial," he says.

Genetics-IVF limits participation in the MicroSort trials to married couples who already have at least one child. And the institute says only two kinds of couples are permitted to use the process: people at risk for one of 350-odd diseases linked to the X chromosome, and families that want to balance their gender mix. People like the couple sitting in front of me, who have three boys and wouldn't mind seeing what a girl is like.

This is not what some in the audience want to hear: They want their one child, and they want it in their favorite flavor. Which leads one woman to exclaim, "I can't believe you're not regulated. You can just decide yourselves what you want to do?"

"It's our choice not to do first children, that's correct," Fugger says.

"What will happen in five or 10 years, we don't know," adds Keith Blauer, the physician who runs the MicroSort clinic. "It'll depend on what society wants."

The American Society for Reproductive Medicine frowns on using gender selection for "family balancing," and the American Medical Association's president is even blunter: "We have to draw a line in the sand," she thunders. But if we've learned anything about technological advances in this century, it's that there's no stopping them, no matter their ethical or moral implications.

The battle against gender selection can only be won person to person. On the argument that surprise does more to improve life than does certainty, I persuaded my wife to join me in gender ignorance throughout her pregnancies, back in a simpler time. In a world sodden with information and reality, can people really not stand even a delicious instant of mystery? No question, it was odd having strangers know which sex our child would be -- various prenatal tests revealed the truth to doctors, nurses, office managers -- but I loved the suspense.

Through a ludicrous combination of non-evidence -- old ladies walking up to my wife on the street to pronounce her bulge male, slips of the tongue by nurses who had seen our test results and then referred to the fetus as "he" -- we were convinced our first child was going to be a boy.

I can think of no moment in my life more thrilling than life emerging -- a shock of black hair, an open mouth with stunning lips, a long torso, and then my wife gasping in shock: "A girl! It's a girl!"

Marc Fisher's e-mail address is fisherm@washpost.com.