Yelena Bonner left the United States tonight after a six-month stay for medical treatment and began her journey home to the Soviet Union, where she will resume her life in exile with her husband, dissident physicist Andrei Sakharov.

At a news conference at Logan International Airport, Bonner said she had received a letter from Sakharov today in which he said that he "can't wait for me to return" and had planted flowers and washed the windows at their home in Gorki in anticipation of her arrival.

Bonner, who appeared healthy and calm as the time for her departure approached, said that what she fears the most is the "unprecedented isolation" of the life that she has shared with Sakharov in Gorki since 1984.

"It is," she said, "a life under constant supervision of the lens . . . hidden cameras are being used everywhere to film us . . . and even more than that, I am afraid nothing but disinformation will be used against us."

Family friends and three grandchildren watched Bonner's plane take off for Paris. Her 86-year-old mother, Ruf, waved at the plane with a small white kerchief.

Her daughter, Tatiana Yankelevich, who accompanied Bonner on the flight, said at the news conference that the family fears "it may be the last time we will see or hear our mother."

Bonner, whose remarks were translated from Russian by her daughter, said that Sakharov, whose health has been in question, "feels more or less all right."

"I want to return to him," she said, "but it is extremely sad and extremely difficult for many reasons."

While she has been in the United States, Bonner has stayed with her son, Alexei Semyonov, daughter and other family members in a Boston suburb.

Bonner is expected to meet with French President Francois Mitterrand and British Prime Minister Margaret Thatcher before traveling to Moscow on June 2. After a few days there, Bonner is scheduled to take a train to Gorki.

When she left the Soviet Union last December, Bonner said she had "made an agreement that if I would be allowed to return, I wouldn't have contact with correspondents, or give interviews or press conferences." That agreement, however, has been stretched over the last few months. Articles by Bonner appeared in The Washington Post on May 11 and 18 and today she wrote for the Associated Press what the news service called a letter to America.

In her meetings with reporters, she was always careful to say she was not giving interviews. She made one exception and granted a television interview after videotapes were obtained from Russian sources showing that Sakharov had been under surveillance in Gorki.

She also attended several public events in support of human rights, including a congressional birthday party last week in honor of her husband, who turned 65. President Reagan declared the day National Andrei Sakharov Day, but Bonner did not meet with the president.

This evening, Bonner said she had no new fears about possible reprisals for what she has said while she has been in the United States. "I believe that anything and everything can be used against me whatever it is," she said.

What she does fear, Bonner said, is a possible recurrence of the severe depression that she suffered after previous visits with family members.

Bonner's mother, who lives here, sat at Bonner's side during the news conference. Bonner told reporters that there had been discussion about her mother, a Soviet citizen, returning to Gorki with her, but she said her mother would remain here.

At the airport, Bonner was given a proclamation by Mayor Raymond Flynn that declared May 28 as Yelena Bonner Day.

Bonner also told reporters that she was bringing Sakharov many photographs. She said she had gotten him, as a gift, a radio-controlled miniature Porsche. "We have no such toys in the Soviet Union," Bonner said with a smile.

In her letter today, Bonner said that she and Sakharov will go to the Gorki post office on the first Monday of each month to contact her children. "I ask everyone concerned with the fate of Andrei Sakharov and myself not to believe any reports about us if they have not been confirmed in a telephone conversation with our children," she wrote.

Bonner, in her letter, also thanked doctors here who had treated her while he was hospitalized for extensive heart bypass surgery and an operation for a clogged artery in her right leg.

Bonner, who was wearing a blue Ultra-suede suit and red blouse with a pearl necklace tonight, said she was feeling well. She noted in her letter that doctors wanted her to return here for further medical tests within a year or two.

After the news conference, Bonner and her mother walked arm in arm down a red-carpeted hall toward the departure gate for TWA Flight 810, which would take her to Paris. Both mother and daughter seemed undisturbed by the harsh lights of camera crews.

Bonner and family members spent about 15 minutes saying their goodbyes in the airline lounge. At 7:10 p.m., Bonner, smiling and composed, began her walk into the jumbo aircraft.

As the plane pulled onto a rain-slicked runway, Bonner's mother stood at a terminal window watching silently. She took her white kerchief, waved briefly and lit a cigarette.