The circumstances under which they died were unclear, with at least one hospital official saying on Monday that at least a few of the patients were dead before the storm.
Another official said the rising temperature in the hospital afterward likely contributed to some of the deaths.
The announcement, which raised Louisiana's official death toll to nearly 280, came as President George Bush got his first up-close look at the destruction in New Orleans and the embattled director of the Federal Emergency Management Agency (FEMA) resigned.
Bush rode through New Orleans in an open truck with the governor and mayor, ducking under low-hanging tree limbs and electrical wires.
"My impression of New Orleans is this: That there is a recovery on the way," Bush said.
The exact number of bodies found on Sunday at the 317-bed Memorial Medical Centre was unclear.
Bob Johannesen, a spokesman for the state Department of Health and Hospitals, said 45 patients had been found; hospital assistant administrator David Goodson said there were 44, plus three on the grounds.
Bush (C) toured the devastated
areas in an open truck
Steven Campanini, a spokesman for the hospital's owner, Tenet Healthcare Corp, said some of the patients were dead before the storm arrived, and none of the deaths resulted from lack of food, water or electricity to power medical equipment. He said many were seriously ill.
Goodson said patients died while waiting to be evacuated over the four days after the hurricane hit, as the temperature inside the hospital reached 41 degrees Celsius.
"I would suggest that that had a lot to do with" the deaths, he said.
Family members and nurses were "literally standing over the patients, fanning them", he said.
Bush, in his third visit to New Orleans since the storm, made his first foray to the streets on Monday, touring the city for 45 minutes.
He disputed suggestions that the government responded sluggishly because the victims were mostly poor and black.
Victims of the devastation
were mostly poor and black
"The storm didn't discriminate and neither will the recovery effort," Bush said.
"When those Coast Guard choppers, many of whom were first on the scene, were pulling people off roofs, they didn't check the colour of a person's skin."
Despite kilometres of flooded streets, there were encouraging signs of recovery.
Nearly two-thirds of southeastern Louisiana's water-treatment plants were up and running.
Louis Armstrong New Orleans International Airport planned to resume limited passenger service on Tuesday.
Forty-one of 174 permanent pumps were in operation, on pace to help drain the still half-flooded city by 8 October.
But that does not mean a quick return to normalcy for residents or for business owners, who were let back in on Monday to assess the damage and begin the slow process of starting over.
Former New Orleans mayor Marc Morial called for a compensation fund for the hurricane victims similar to the fund created for victims of the attacks of 11 September 2001.
It will be at least three months before New Orleans's public water system is fully operational, said Sergeant John Zeller, a California National Guard engineer working on the systems.
Some homes have running water, but it is mostly untreated Mississippi River water - for anyone wanting a bath, "it's like jumping in the river right now", he said.
Others will find their homes are not even liveable.
Not a single structure is left standing in Hopedale, southeast of New Orleans, St Bernard Parish President Henry Rodriguez told displaced residents gathered at the state capital.
Parish Councilman Craig Taffaro said no one should expect to live in the parish again before next summer. Before Katrina, its population was 66,000.
Many homes remain unlivable
FEMA expects to provide temporary housing for 200,000 hurricane victims for up to five years, most of them in Louisiana.
It plans to use trailer homes to create "temporary cities", some with populations up to 25,000, said Brad Fair, head of the FEMA housing effort.
"This may not be quite on the scale of building the pyramids, but it's close," Fair said. He had no cost estimates.
In New Orleans, a contractor was back at one of the previously repaired levees after water was found to be seeping through.
Brigadier-General Doug Pritt of the Oregon National Guard described it as a minor leak.
Insurance experts have placed
insured losses at $40 billion
Insurance experts have doubled to at least $40 billion their estimate of insured losses caused by Katrina.
Risk Management Solutions Inc of Newark, California, put the total economic damage at more than $125 billion.
Lawmakers in Washington proposed some tax changes on Monday to help the victims, such as letting them tap retirement accounts without penalty and encouraging donations of cash, food and school books.
According to the White House, Bush's former FEMA director Mike Brown was not asked to resign.
Brown said he decided to step down "in the best interest of the agency and best interest of the president".
He was quickly replaced by R David Paulison, a top agency official with firefighting experience.
Brown had been vilified for the slow federal response to the hurricane, already considered the nation's costliest.
In New Orleans's central business district - which includes oil and gas companies, hotels, restaurants, banks and brokerages - business owners were issued passes on Monday to return to parts of the city to retrieve vital records and equipment.
New Orleans Metropolitan Convention & Visitors Bureau president J Stephen Perry said Katrina cost the city about 100 to 200 major conventions. But he expected the tourism industry to be among the first to bounce back, since the French Quarter and many hotels suffered little damage.
"The really positive thing long-term is, the core of our infrastructure of the $5 billion to $8 billion tourism industry remained intact," Perry said.
"As odd as it may sound right now, we are optimistic that this recovery is not only going to happen, it's going to happen well and we're going to have a great city going again."