Conrad Hilton, who died of pneumonia Wednesday night at St. Joseph's Hospital in Santa Monica, Calif, at the age of 91, was known as "the biggest hotel man in the world."

He got into the business in 1919 when he invested $5,000 to buy the Mobley, a hostelry in Cisco, Tex., then an oil boom town. At the time of his death, the Hilton Hotel Corp., of which he was board chairman, operated 185 hotels in the United States and had an estimated value of $500 million.

In his time, Mr. Hilton built and operated hotels in major cities and watering places from London and Paris to Istanbul, Hong Kong and Acapulco. In 1967, he sold Hilton International, a subsidiary of Hilton Hotels, to Trans-World Airlines, now Trans World Corp. Today, Hilton International operates 75 hotels in 45 countries.

Mr. Hilton's propoerties included the Waldorf-Astoria, the New York Hilton and the Savoy in New York; the Capital and Washington Hiltons in Washington (from 1946 to 1956, he also owned the Mayflower in Washington); the Palmer House and the Conrad Hilton in Chicago; the Shamrock Hilton in Houston; the Denver Hilton, and the Beverly Hilton in Beverly Hills, Calif.

Some of his acquisitions were spectacular for their size as well as for their prestige. In 1954, he paid a reported $76 million for the Satler hotel chain. He founded the Carte Blanche credit card company.

Mr. Hilton, who lived in a 61-room house in Bel Air, Calif., was known as "Connie" to his friends. He was said to be friendly, gregarious, fond of swimming, horseback riding and dancing, and addicted to work.

His devotion to work was said to have been a factor in the ending of his first two marriages. His first wife was the former Mary Barron, whom he married in 1925. His second wife was the Humgarian actress Zsa Zsa Gabor, from whom he was divorced in 1947. At the time of his death, he was married to the former Mary Frances Kelly.

On his business success, Mr. Hilton once told Newsweek magazine how he decided whether to buy a hotel.

"First, it must be the best hotel in the community," he said. "Then, we examine its statements. We inspect the property with an eye toward the cost of conditioning and promotion and especially the prospects for digging for gold."

"Dig for gold" was a mixim of Mr. Hilton. It meant converting unused space -- parts of lobbies, for example -- into revenue-producing areas, such as stores.

Another maxim was "minimax:" minimum cost for the maximum in service and hospitality. Guests at Hilton hotels would be "forward-passed" to other Hiltons.

On his work habits and amusements, Mr. Hilton told Newsweek that he "found it a healthful rule to try to close my business day with the setting of the sun. I have learned to put down and set away whatever the day has brought and I have learned to find relaxation.

"You can put it down in plain talk," he continued. "Conrad Hilton likes to dance, and most particularly, he likes to dance with a pretty girl. There must be something wrong with people who don't."

It was Mr. Hilton's practice to open new hotels by performing with a pretty partner the varsoviana, a European dance said to have been imported to Mexico by the Emperor Maximilian.

As his operations expanded overseas after World War II, Mr. Hilton came to think of himself and his work as playing a role in the Cold War. "People get together in our hotels and get along with one another," he once said. "These hotels are examples of free enterprise that the communists hate to see."

Associates said Mr. Hilton remained active in the direction of the business empire he founded until his death. He was admitted to the hospital last Sunday suffering from congestive heart failure.

Conrad Nicholson Hilton was born on Dec. 25, 1887, in San Antonio, N.M. He was the second of eight children born to August Holver Hilton, a Norwegian immigrant, and Mary Laufersweiler, whose family was German.

His father, the leading merchant of San Antonio, operated a general store, a livery stable, a bank and other businesses.

The family also rented out spare rooms in the house to travelers. Young Conrad used to meet the trains stopping at San Antonio and guide salesmen and others to his home, where they could get meals and lodging for $1 a day.

"That was the first Hilton Hotel," he used to say. "There were eight of us kids and as each of us grew up and went away to college, our rooms would be rented."

Young Hilton was a student at the New Mexico Military Institute and at the New Mexico School of Mines. In 1913, he helped his father found a bank. He also served a term in the New Mexico legislature in 1912-13.

During World War 1, he was a lieutenant in the Army and served in France. His father was killed in an automobile accident while young Conard was away at war. When he came home, the son took over the family enterprises.

According to one story, he went to Cisco, Tex., in 1919 to expand his banking business and wound up buying the Mobley there when the banking negotiations bogged down.

Throughout the 1920s, Mr. Hilton bought, operated and sold various hotels, most of them in Texas. His expansion was stopped cold by the onset of the Great Depression of the 1930s, and he lost four of his properties. But he held onto five of them by making economies, such as shutting off some rooms or even whole floors of rooms. In 1935, money from oil leases allowed him to pay his most pressing debts.

In 1939, he built a hotel in Albuquerque, N.M. It was the beginning of a series of acquisitions that never really stopped. In 1939, he also acquired a hotel in Long Beach, Calif. In 1942, he bought the Roosevelt and the Plaza hotels in New York. In 1945, he bought the 3,000-room Stevens in Chicago and renamed it the Conrad Hilton. In the same year he also took over the Palmer House in Chicago.

In 1942, he made a deal for which he was to become famous in business circles. He bought bonds of the Waldorf-Astoria Corp., operators of the famous hotel in New York, with a face value of $500,000 for about $22,000. He later sold them for more than $400,000.

In 1949, he bought the Waldorf-Astoria itself. The hotel was the acme of elegance in the United States and Mr. Hilton said he was proud to be its owner.

In the 1950s and 1960s, Mr. Hilton devoted most of his time to overseas expansion. He also wrote an autobiography, "By My Guest," copies of which were placed in Hilton hotel rooms with a note inviting guests to take one home for free. In 1966, he stepped down as president of the Hilton Hotels Corp., but continued as chairman of the board. A son, Barron Hilton, was elected to succeed him as president.

Besides his son Barron, Mr. Hilton's survivors include his wife, whom he married in 1976, and another son, Michael Eric, a vice president of Hilton who lives in Houston, and a daughter, Francesca, by his marriage to Miss Gabor, of Los Angeles.

Another son, Conrad Nicholson Hilton Jr., an executive of the corporation who was the first husband of actress Elizabeth Taylor, died in 1969.