Fair Oaks Hospital officials were patiently waiting yesterday to start an early-morning ceremony marking the hospital's grand opening when an unexpected visitor arrived.

"Is the medical center open yet?" the man asked. "I've got a real bad case of Montezuma's revenge."

The 37-year-old Chantilly man unceremoniously became the first patient of Fair Oaks, a $27 million, 160-bed hospital designed by the Fairfax Hospital Association to replace Commonwealth Hospital in Fairfax City.

The new 179,000-square-foot facility on Joseph Siewick Drive in western Fairfax County is more than double the size of the 20-year-old, 127-bed Commonwealth, which was in its final day of operation yesterday.

Despite the unexpected patient, who asked not to be identified, the dedication at Fair Oaks went on as planned 30 minutes later, followed by the four-hour, formal transfer of Commonwealth's 21 remaining patients.

A blue van escorted by two motorcycle officers arrived first with Irene Anderson, 48, of Sterling, who originally was to be the hospital's first patient.

She was greeted by hospital administrator Steven Brown with a bouquet of flowers, and taken by wheelchair to the hospital's best accommodations on the third floor, where she would stay for free.

"This is the nicest thing that has happened to me," said Anderson, who is being treated for multiple sclerosis.

"I'm excited. This is a once-in-a-lifetime thing."

Anderson's private suite was equipped with a color television, vidocassette recorder, rose and a monogrammed robe with "The Oaks" inscribed.

"This is like a hotel," she said.

Although he praised the new hospital, Ernest Luther, a 64-year-old pharmacist who had been with Commonwealth since it opened in 1967, said yesterday was also a time to remember.

"The Christmas parties {at Commonwealth} have always been great and we've had two or three marriages here," he recalled.

The first couple was married at the hospital because the bride became ill just before the wedding, Luther said.

The other wedding he remembered was held at the hospital because the mother of the bride had an accident on the way to the ceremony.

"We had punch and cookies and snacks for the wedding party," Luther said. " . . . . We've had some good memories here, but we will build up some over there, too."

In the Commonwealth's geriatric unit, staff members bought a card to celebrate the end of an era. Inside, the signed card read: "Good-byes are never easy and that's especially true when the building we are leaving is as wonderful as you."

William Floyd Baughman, 78, said he was happy with the way he had been treated at Commonwealth and expected to be treated the same at Fair Oaks.

However, Baughman, who has cancer, added that when he leaves the new hospital he hopes he will be in such good health that he will never have to return.

As for the hospital's first patient, Dr. Richard A. Steinberg, a Fair Oaks emergency room physician, reported that he had prescribed some medicine and the patient was doing well.