CHICAGO -- Georgia O'Keeffe's "East River From the Shelton," a painting that directors at the Art Institute of Chicago thought had been misplaced for 20 years, has been formally -- and very carefully -- classified as "lost" by the museum.

Art Institute officials stopped short of calling the work "stolen" but acknowledged that, with the painting nowhere to be found, they have notified local, federal and international authorities about the loss.

Previous directors believed the work, valued at between $250,000 and $500,000, had been mislaid in storage of the permanent collection.

But James N. Wood, institute director since 1980, revised that opinion when the painting did not turn up in a recently completed inventory of the 1,500 pieces in the museum's department of 20th-century paintings and sculpture.

"I became aware of the situation in 1987," Wood said Monday, "when our new 20th-century curator, Charles Stuckey, was preparing for an O'Keeffe show and could not find the painting. We then initiated the inventory. Now that it is finished, I think it highly unlikely we will find the painting on the premises.

"We felt it was important to complete the inventory before contacting authorities," Wood said, "but now we have notified the FBI, Interpol and the International Foundation on Arts Research in New York. We also have begun negotiations with Huntington Block, our insurer."

No other works were found to be missing from the 20th-century painting and sculpture collection, but institute officials recently began an inventory of the museum's entire holdings, about 300,000 pieces, of which only about 10,000 are on view at any given time. The full inventory, which will be computerized, will take several years.

In 1978, three paintings by French artist Paul Cezanne, valued at $3 million, were stolen from a second-floor storage room by an institute employee. Officials recovered the works five months later.

That robbery prompted installation of a state-of-the-art security system, including sophisticated monitoring and control of storage vaults.

"It is a mistake to think the painting has absolutely been stolen," Wood said. "One has heard of objects being lost in museums, or even going out with the trash. So it still might be found here or in the hands of an innocent person. It's meaningless to speculate. One can only hope."

O'Keeffe's 12-inch-by-32-inch canvas, painted in 1926, depicts a view from a window of the hotel where the artist and her husband, photographer Alfred Stieglitz, lived in Manhattan.

The painting probably appeared in her first exhibition including New York scenes at the Anderson Galleries in 1926. Stieglitz's estate lent the work to the Art Institute in 1949, and it entered the museum's permanent collection as a gift of the artist in 1955.

Institute records show that the painting was returned to Chicago from a retrospective exhibition at Houston's Museum of Fine Arts in 1966. But when the Whitney Museum of American Art in New York requested the work for a retrospective four years later, institute staff members were unable to locate it.

The artist, an alumna of the School of the Art Institute, knew of the problem. Wood said Monday he assumed museum officials informed her of the situation during the 1970s. O'Keeffe, who died in 1986, is recognized as one of the giants of 20th-century American art.

In the early 1920s, O'Keeffe watched the Shelton being built on the corner of Lexington Avenue and 49th Street. When she and Stieglitz outgrew a studio on East 58th Street, they moved to a two-room suite on the 30th floor of the hotel, where they remained for a decade.

"I had never lived up so high before and was so excited that I began talking about trying to paint New York," O'Keeffe wrote a half-century later.

She painted views from the north and east windows of the suite for only five years. They are not what has come to be known as "typical" O'Keeffes, either in style or subject matter. Her best-known works are early abstractions, large-scale flowers and semi-abstract paintings showing the influence of her time in the American Southwest.

"There were several 'East Rivers,' some with snow, and a large one with low buildings against the river," O'Keeffe wrote.

A pastel of the same subject, "East River, New York, No. 2" (1927), appeared in the memorial exhibition of her work in Washington and New York, but it did not come to the institute with the rest of the show in 1988.