EASTERN SAUDI ARABIA — Capt. Richard Mayo, commander of Navy Fleet Hospital 5, put his staff on combat-ready status as soon as the ground war started, preparing his vast, canvas 500-bed hospital for a flood of expected casualties.

And then the 60 doctors and 900-plus staff of the unit waited.

And waited.

Nurses and orderlies played Pictionary in a corner of the emergency room. Others read, wrote letters, studied medical texts and wolfed down another prepackaged, foil-wrapped, stomach-churning MRE.

Finally, after 17 hours, the first patients were choppered in. Marine Cpl. Martin Wilcox, of Seattle, had a bullet in his left arm. And Marine Cpl. Mark Zahuranec, of Quantico, Va., had scalded his right arm in a cooking accident.

Someone else had a flesh wound, another a twisted ankle from jumping in a foxhole.

And so it went in the elaborate field hospital system set up in the desert to provide surgery, intensive care and more for the war that mostly wasn't. After two days of fighting, allied casualties have been extremely light, and although the war isn't over yet, medical teams were delighted.

"The less we see, the better," said Navy Lt. Cmdr. John Halpern, an emergency-room physician who usually works at Lincoln Hospital, located in a drug-ravaged, gang-plagued neighborhood of the South Bronx. Most nights there, he said, were rougher than the start of the biggest invasion since D-Day.

"There is nothing like Lincoln," said Halpern. "I'm relieved and happy. Our jobs are to be here just in case."

Another hospital unit, which had 3,000 beds in its regional field units, had treated fewer than 30 casualties. Several weeks ago, by comparison, a hospital serving a U.S. Air Force tactical fighter wing here reported treating 600 patients suffering from acute food poisoning.

This war saw doctors prepare special field surgical units, planning to put MASH units virtually on the front lines. An eight-tent hospital unit with four operating tables, which took 18 to 24 hours to set up in Vietnam, now can be trucked into the field and start operating within two hours.

"Once in place," said Lt. Col. Philip Elledge, who developed the concept, "96 doctors, nurses and enlisted personnel will be able to supply maximum emergency surgical resuscitation and primary treatment to as many as 225 people in 24 hours."

Only if they need to, however. Not a single soldier has been killed in the entire attacking 18th Airborne Corps, which poured into Iraq over a stretch 165 miles long.