The music world is about to hear a sound it has not heard in 18 years: the sound produced by the right hand of Leon Fleisher.

On Sept. 16, in the new Joseph Meyerhoff Symphony Hall in Baltimore, Fleisher, one of the great pianists of our time, will make the most dramatic return to the piano since Vladimir Horowitz came out of voluntary retirement in 1965. Fleisher's concert, with the Baltimore Symphony Orchestra at the gala opening of its new hall, will be broadcast live on WETA-FM and taped by the Maryland Center for Public Broadcasting for nationwide stereo simulcast on Sept. 25 (locally, at 9 p.m. on Channels 22 and 26 and WETA-FM).

Fleisher's right hand was silenced by a neurological problem in 1964, when he was 37. He recalls it as "the gradual onset of a syndrome that they actually call writer's cramp -- an involuntary curling under of the fourth and fifth fingers and an inability to get them straightened out in time to play the next notes." A similar problem has also disabled the right hand of Gary Graffman, another American pianist of international stature and a friend of Fleisher's. "Gary is watching my progress very carefully, and we are in constant communication," Fleisher says. "He is seriously considering my treatment, but at the moment he is going a different route. He is now in the middle of another series of procedures."

Before this problem hit him, Fleisher was one of the most acclaimed pianists in the world and one of the busiest. Perhaps too busy. "I had been doing an awful lot of playing under pressurized conditions," he recalls. "During the last year before I stopped, I played 21 times in New York City alone. I had worked my muscles into a state where they no longer relaxed between performances, a state of chronic tension. I suffered from a sort of petrified muscles." Remembering those days, Fleisher is planning to make his comeback very slowly. He has accepted only two piano engagements for the coming season and is much more definite about his conducting plans, including a Stravinsky program Oct. 2 at the Terrace Theater to open the season for the Theater Chamber Players.

Even after his long absence from the concert stage, some of Fleisher's records -- notably the sets of Beethoven and Brahms piano concertos with the Cleveland Orchestra and George Szell -- are still in circulation and are regarded by many critics as standard-setters. But in recent years, his live piano performances have been limited to the meager repertoire for the left hand: chiefly concertos by Ravel and Prokofiev, a set of "Diversions" with orchestra by Benjamin Britten and a quintet with strings by Franz Schmidt. He has continued to win critical superlatives in this music.

During the years of his disability, Fleisher also became active in chamber music and conducting, as music director of the Theater Chamber Players in Washington since the group's establishment in 1968, associate conductor of the Baltimore Symphony for the past five years and music director of the Annapolis Symphony for 12 years. He has also been a faculty member of the Peabody Conservatory in Baltimore since 1959.

Fleisher has been undergoing therapy on his right hand since January, following surgery for a condition called "carpal tunnel" related to wrist flexibility. The therapy, twice a week at the Northern Virginia Myotherapy Clinic in Springfield, involved "a lot of detail work on the hand, back and shoulder" and "a series of strength and stretch exercises to rebuild the muscle efficiency," according to Nancy Shaw of the clinic, a therapist who has now become a fan of Fleisher's music. "I was at the rehearsal last week and it went extremely well," she said.

Rumors that Fleisher was thinking of a return to the standard repertoire have been circulating in the music community since last spring. But his final decision was made only a few days ago, long after the orchestra confidently announced that he would play Beethoven's Fourth Piano Concerto -- one of his specialties in the old days -- at its gala opening. Fleisher has decided at the last minute to substitute Franck's Symphonic Variations. He will probably get back to Beethoven later, he said, but "at the moment, I'm not planning or projecting; I've got to see how things go for a while."

He still has not accepted any other requests for public appearances as a two-handed pianist, except for one in March, at the Kennedy Center's Terrace Theater, with the Theater Chamber Players.

"It's still not quite the way it was before," he said. "I have tried to get in a lot of early rehearsals, while they were tuning the new hall, and I think it will be easier by the time Sept. 16 rolls around."

His therapy uses some of the techniques of rolfing -- deep, intensive massage that deliberately deprives some muscles of oxygen for up to eight seconds, making it impossible for them to remain in a state of contraction. "They use elbows, knuckles and fingers with great traction and great weight," he said. "They go up and down each muscle in a very painstaking and painful process, using deep and heavy pressure the entire length of the muscle until it loosens up."

Fleisher's withdrawal from the concert stage came as he was on the brink of a tour of the Soviet Union with George Szell and the Cleveland Orchestra. "I had just finished a performance of Mozart's K. 503," he recalls (that's the 25th Piano Concerto in C, and their recording of it is still on the market), "and we decided I simply could not do it." His first reaction was a depression that lasted several years. "Then," he says, "I managed to pick myself up. One thing that contributed to my recovery was friend and fellow pianist Dina Koston's idea of starting the Theater Chamber Players. She came to me and discussed the idea, wanted to know whether it seemed feasible and I might become the artistic director. She invited me -- practically forced me -- into a conducting role. After my first year with the Players, having gotten my feet wet as a conductor, I was ready to take over the directorship of the Annapolis Symphony."

Fleisher was familiar with orchestral music before becoming a conductor. "I used to play a lot of the orchestral repertoire in four-hand piano arrangements," he recalls, "and sometimes I would play the orchestral parts of concertos in piano reduction for friends who were practicing the solo part." One such friend was Graffman, who won the Leventritt Competition in 1949 playing the Brahms D-minor concerto while Fleisher played the orchestral part on a second piano. In his autobiography, Graffman calls his friend "the Fleisher Philharmonic" and recalls that some of the judges wanted to give Fleisher a prize, although he was not competing. He won a comparable prize -- the Queen Elisabeth Competition in Brussels -- three years later.

While he was still a student of Artur Schnabel, Fleisher also became a protege' of Pierre Monteux, who publicly called him "the pianistic find of the century" and began engaging him as a concerto soloist in New York and San Francisco when he was 14 years old. He used to spend summers in Maine, where Monteux ran a school for conductors, and there he learned the technique, although Monteux would not allow him to conduct the student orchestra.

"I would ask him, 'Can I conduct?' " Fleisher recalls, "and he would say, 'No. Once you get a stick in your hand, you will never let it go.' I think he was right. I don't want to give up conducting. No matter how negative my experience has been, it was a soil for further growth and I'm terribly grateful for that opportunity. Being a pianist is really a very solitary occupation, but compared to being a conductor, in a way, it is very luxurious. You can practice forever, try things this way and that at your own pace, go out for a glass of water and come back when you're ready. With an orchestra, economics and time limits come in; you have to diagnose a problem and come up with your prescription immediately."