Monika Weiss, 29, from San Francisco, California, sits in the Preecha Aesthetic Institute in Bangkok comparing two different kinds of vagina. "They can invert what's there, and put it inside. It's called a penile inversion. That is about US$8,000 dollars here in Thailand," she says. "They have another procedure, where they use a piece of your colon to create a vagina." That costs $20,000, Weiss says.

"I would probably go for the colon one, because it is a better quality. Certain complications are lessened with that, and it's more durable. And I've been told that it feels more real."

Weiss has come to Preecha Aesthetic Institute to place her identity in the hands of Thailand's most famous transgender surgeon, Dr Preecha Tiewtranon. She is one of the lucky ones. When it comes to the complex world of sex changes, turning a man into a woman (using a penis to create a vagina) is relatively easy. Turning a woman into a man is far more complicated.

But it's all in a day's work for Preecha. "I have done more than 3,500 operations in 30 years," he says at the modern, three-story, white-walled institute on upmarket Thonglor Road.

The process for Weiss, should she decide to go through with it, is more often than not a great success. After one of Preecha's 15 staff surgeons slices off the male organ and plumps the breasts with implants, the newly created woman can usually enjoy sexual relations and climax, Preecha says. "Eighty percent have a relationship with a man. The rest become so-called lesbians. About 80 percent of the patients are able to orgasm," Preecha says, noting that factors like age and hormone use also play a role in reaching climax.

Weiss admits the possible loss of her ability to climax is a concern. But like thousands before her, the urge to receive the 'correct' body often overpowers these lingering anxieties. And Preecha and his institute are benefiting handsomely.

Monika Weiss underwent a partial sex change to become a woman and is considering a future penis removal.Cost-friendly gender reassignments

The relatively low cost of surgery in Thailand is why so many patients choose Bangkok for sex change operations. Males pay US$9,000 to $20,000 or more to become females, and comprise the overwhelming majority of Preecha's patients.

Preecha charges female patients US$12,000 for a one-inch penis, which includes deflating the breasts and removing the uterus and ovaries. A longer phallus boosts the total cost to almost US$20,000 dollars.

Business is booming, especially because his clinic also alters noses, eyelids, and covers other procedures, including liposuction. In fact, Preecha said he will open another clinic in Hanoi, Vietnam, later this year, and also hopes to expand to Dubai.

Weiss underwent her partial male-to-female change at Preecha's clinic in January. "I paid one-third of what I would have paid in America for the same procedures," she says while recovering in Bangkok. "It was US$10,700. This was basically my first semi-permanent move to see if being a transgender, being a woman, was the right move for me. I've been on female hormones for 11 years, and most people in San Francisco still take me as male, so I was hoping this would push me, and that most people would start perceiving me as female. We shall see."

One of Preecha's surgeons gave Weiss breast implants, a new nose and other procedures to feminize her face. Weiss says her decision to go for full sexual reassignment surgery, to remove the penis and make it into a vagina, "depends on money at this point, and how all this works out for me."

Dr. Preecha's institute is an outpatient clinic. This chair is used for genital observation, though he performs sex change surgeries at a nearby hospital.Female to male -- not so easy

Displaying excruciatingly graphic medical photos on his computer's screen, Preecha explains why it is much more difficult to turn a woman into a man. He notes that the medical and cosmetic problems involved with the procedure convince most women to opt for a very small penis.

"To do the long penis involves a very big operation, and quite a lot of mutilation, because the patient will have big scars on the side of their belly, or on the arm, or the thigh, from the skin graft or the skin flap. Today, we have more and more patients wanting the so-called small penis, or the 'micro penis.' We usually elongate the clitoris, and the patient is able to have about one inch of a penis. He can stand up to urinate in the toilet. They still can stimulate themselves with masturbation. They can achieve sensation."

Sexual intercourse, however, is not possible with a one-inch penis. "Of course they cannot do penetration. Too short. Only rubbing around. But they still can enjoy sexual pleasure."

The other advantage is "no scar at all." Skin grafts, from elsewhere on the body, are not needed if an entire phallus is not required. "Longer, you need a lot of skin. Average size? We can do six inches, eight inches. But average is about four. It depends on the wish of the patient."

To make a penis longer than one inch, a tubular shape must be added, which can function in ways similar to the real thing, albeit with some numbness.

"The reconstructed penis has no sensation. They are able to stand up and have urination. They are able to rub it, and feel like they are getting the female partner excited. And they are able to get the hardness, by using the silicone implant, and have the sexual intercourse."

A Thai transsexual in a Bangkok slum displays her implants to her female friends, who are sex workers.Why switching sex under a surgeon's blade takes time

If you are a woman who wants to become a man, "you have to go through multiple operations. There is no such thing as one stop, like male-to-female," says Preecha. "Male-to-female you can get everything in two weeks, and then you go back home, recuperation for another two weeks, and then you are completely a woman."

Turning a female into a male, however, increases the patient's risk, especially if a woman requests a penis long enough for penetration. "The silicone could extrude. When you have the extrusion back home, the cost is so high to go to see the doctor," Preecha says.

The Thai government recently changed the law on transgender operations and now requires Thai patients to wait at least one year before undergoing a sex change. Sex changes are outlawed completely before the age of 18, and for those aged 18 to 20 parental consent must be obtained.

In response to the new law, Preecha says he sends his Thai patients to two psychiatrists, to confirm the individuals qualify. Most of Preecha's patients, however, are men from America, Australia, Europe, China, Japan, South Korea and the Middle East, who want to become women.